Bug: Assertion when moving a gridbagsizer item into a sizer (via right-click menu)

[KeyWorksRW]

Description:

In Debug builds, if you select an item in a gridbagsizer, right-click it and move it into a sizer, you get an assertion error about it not being correctly added to the gridbag. It does then work and get added correctly, but we should at least figure out why the assertion is happening and see if we can prevent it.