In the case that no color block is selected in the palette, using the [color] → [Colorize with selected color] command in the [Font Overview] window will cause FontCreator to become unresponsive, and then it cannot be closed in the normal way.
I was able to reproduce the bug if a glyph edit window was open and if the Colour mode was enabled in the glyph edit window.
One can escape from the problem by browsing to a monochrome glyph (which will be blank while Colour mode is enabled) or by disabling colour mode, or by clicking Undo.
Thank you for answering my question.
Once FontCreator is in an unresponsive state (or suspended animation state), the entire FontCreator interface area cannot respond to mouse and keyboard operations, so the FontCreator process can only be stopped by forcing the end of the FontCreator process through the task manager of the operating system.
This bug will be fixed with the next upcoming release.