fix: Don't use a key for the ErrorBoundary in a Panel

[mofojed]

• Changing the key whenever the children changed resulted in the key changing every render, which is not what we want
• Don't bother with an error key; when the widget is re-opened, there's already a new key at a higher level so the ErrorBoundary gets reset

[mattrunyon]

This will need to be cherry picked to ui-0.15