UI: Fix crash with adding source

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Bug introduced with https://github.com/obsproject/obs-studio/commit/adba393ca85fba19ed1bf6cd825ab8188beb2d16

OBS would crash when adding a new source because the source tree item's
widget would be NULL in the sizeHint function when the list item is first
created.

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500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f