Prerequisites:
Superproject with staged file
Submodule with uncommitted change (staged or unstaged)

Steps:
1. Commit in superproject
2. SourceTree checks submodules, prompts that submodule has changes.
3. Click "Open" in submodule popup.
4. A "Commit" popup appears which misses a lot of text fields, buttons (Commit/Cancel) are unresponsive - see attachment. It is not obvious whether this popup relates to the superproject or the submodule.

I have to close the repo tab and reopen to continue.

Workaround:
1. Manually commit change in submodule
2. Commit in superproject (with or without the submodule ref change)
3. OK