Close Tab doesn't close the last tab

Nightly Build "4f8cdbb" on Windows XP SP3.

Steps to reproduce:
Open MuseScore with no items open (Edit, Preferences, General tab, Program Start, start empty).
Open any MuseScore file.
Click the Close Tab (X) button on the tab showing the file name; or click File, Close; or press Ctrl+F4.

Result:
The contents of the tabbed window and the Close Tab button is cleared, but the tab itself (and the file name) is not (see image). The tab does fully close properly when another user action which opens a dialog (e.g. Open, Style, etc.) — or which requires a screen redraw — is taken.