Null references and need to relog when rebuild

[AurL]

When editing any script in Unity, it does a rebuild of every scripts and raises Null references exceptions?
It seems to happen because it recompiles SceneToGltfWiz.cs script while it's still attached to the exporter game object.

Potential fix: check if exporter is null in OnGUI() and recreate it if needed. It avoids the exporter to close and reopen, needing the user to relog with oauth.