Promoted widget canvas image preview inside subgraph disapears on tab / wf change

### Custom Node Testing

- [x] I have tried disabling custom nodes and the issue persists (see [how to disable custom nodes](https://docs.comfy.org/troubleshooting/custom-node-issues#step-1%3A-test-with-all-custom-nodes-disabled) if you need help)

### Expected Behavior

When promoting widget canvas image preview inside (for KSampler) inside subgraph that widget is present in subgraph widget.

### Actual Behavior

When promoting widget canvas image preview inside (for KSampler) inside subgraph that widget is not always present, it disappears when changing workflow tab or doing changes inside subgraph.

### Steps to Reproduce

Create subgraph with KSampler -> promote canvas image (options available only after first generation, at least on my side) -> go out of subgraph and confirm if image preview widget is visible
Add new workflow in Comfy or change tab to other workflow and go back to original
Preview widget is not visible in subgraph.

### Debug Logs

```powershell
N/A Ui issue
```

### Other

_No response_

Reproduce the issue by creating a subgraph with KSampler, promoting the canvas image preview after a generation, then switching workflow tabs or editing the subgraph. Start by tracing how the promoted widget is restored when returning to the workflow; no file or test is named in the report. Done means the preview remains visible after those tab and subgraph changes.

Written by the indexing model from the issue text.