Clicking 'styleSheet' button in Property Editor (of Form Designer) causes Qt Creator to hang indefinitely

When in Form Designer (of Qt 5.12, Qt Creator 4.8.1), I have tried to edit the styleSheet of a selected widget via:

1) clicking the 'styleSheet' button in Property Editor (has "..." button text).

2) right-clicking on select widget and then clicking the 'Change styleSheet' option.

Both methods are supposed to open the styleSheet dialog. Instead, they cause my session of Qt Creator to hang indefinitely. The program is unresponsive to any button clicks or key presses. It can only be closed via Task Manager. This issue affects all previous projects and new projects (see attached zipped files for an empty project that is affected). While Qt Creator is hanging, I do not detect any CPU or memory abnormalities in Task Manager.

Uninstalling and reinstalling Qt resulted in no success. The styleSheet features mentioned above were working just fine for me previously with the same version of Qt.