CompilationDatabaseProjectManager: crashes qtcreator when the project's .json file is modified

The m_fileSystemWatcher triggers a reparseProject when the compilation_database.json is modified. This often leads to crashes with various different backtraces (see attachment).

To reproduced load a compilation_database.json project (6000 lines, but also shows up when reduced to 30 lines), open a .cpp file from it and touch the .json file repeatedly while Qt Creator is focused and in use: while sleep 1; echo .; do touch compile_commands.json; sleep 0.2; touch compile_commands.json; done.

This also happens when the sleep-interval is made large enough for the previously triggered project parsing to finish.
 
On the master branch (dd9086530f4b6164c452a7849b3ba37cb063bea1).