To reproduce:
- Enter play mode
- Try the "Show / Hide" button on the text panel (this will work as intended hiding/showing the canvas).
- Exit play mode
- Re-assign the TextPanel canvas to the On Click () event on the "TextPanelButton" (drag it on)
- You will see the (Canvas) reference changed to an Object reference instead
- Enter play mode
- Try the "Show / Hide" button again
- Observe the Null exception error

Workaround is possible by dragging the canvas component to the OnClick() event instead of the whole object