welcome to the Atlassian Community.
Some time ago there as been similar threads here in Community which suggested (first of all) to reinstall SourceTree. In case you haven't done that - it might be worth a try.
For further reference I'll link the thread, maybe it will be of help.
I recognized that you seem to use a older version (although I was not able to identify a version 3.3.9). The latest one as of writing is version 4.0.1.
Maybe upgrading to a later version also helps.
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