FileMerge quits immediately after launching from SourceTree

Question

I regularly use Atlassian SourceTree (on Mac OS X) to launch FileMerge to resolve git merge conflicts. Out of the blue it has stopped working: when I right click and select

Answer 1

This also works to resolve the message "xcode-select: error: tool 'opendiff' requires Xcode, but active developer directory '/Library/Developer/CommandLineTools' is a command line tools instance":

sudo xcode-select -s /Applications/Xcode.app/Contents/Developer