Fix swapping from linear to normal workflow

There was an attempted fitView with 0 visible area
Not super happy with location of fix
- FitView shouldn't cause total loss of functionality if called from
  invalid state
- As previously discussed, this fitview call should probably be moved to
  only apply on newly loaded workflow, not restored workflows

src/scripts/app.ts

@@ -1227,6 +1227,8 @@ export class ComfyApp {
           // Fit view if no nodes visible in restored viewport
           this.canvas.ds.computeVisibleArea(this.canvas.viewport)
           if (
+            this.canvas.visible_area.width &&
+            this.canvas.visible_area.height &&
             !anyItemOverlapsRect(
               this.rootGraph._nodes,
               this.canvas.visible_area