carto crashes on selecting a tool when no image opened

[Rafal Wokacz]

When there is no image opened and one attempts to select a brush or any other tool from Tool Box, then application crashes with
message:
Qt has caught an exception thrown from an event handler. Throwing
exceptions from an event handler is not supported in Qt. You must
reimplement QApplication::notify() and catch all exceptions there.

terminate called after throwing an instance of 'std::bad_alloc'
  what(): std::bad_alloc

[Rafal Wokacz (rawo)]

Exception is thrown at mainwindow.cpp, line 192:
Q_CHECK_PTR(sc);