To reproduce:
1) Go to Tools | Options | Version Control | Git and uncheck "Show diff side-by-side"
2) Load analogclock project from qt examples
3) Do some change to main.cpp and save it
4) Do: Tools | Git | Current Project | Diff Project "analogclock". Diff output should appear in editor. Don't change the current editor.
5) Do: Tools | Git | Current File | Diff of "qt-48". Creator crashes. Notice, that "Current File" menu should be disabled in that context (like in creator-27)