Theme Editor: fix leading styleboxes / main styles

Recent changes in Godot cause the theme editor to become hidden when
editing a child resource. This causes a crash when editing style box
resources marked as "main styles" (= leading styleboxes in the code), as
they try to reference the currently edited theme.

This commit works around the issue by permitting the Theme Editor to
keep a reference to the most recently edited Theme. Furthermore, it adds
an assertion to avoid a similar crash in the future.

Long-term, the workaround should probably be removed when the theme editor
is fixed to remain visible while editing child resources, but I'd keep
the assertion.

0 files changed, 0 insertions, 0 deletions