How to reproduce:
If I create a local branch, let's call it "a", (and don't check it out) and filter my graph based on "a" and some other branches, and I then delete "a", the graph window turns black.
This is unexpected to me, as I should be able to see all branches that I chose for the filter.
So either "a" should be ignored as it does not exist, or it should be removed as a branch selected for the filter when I delete it.