Can't switch between source/project tabs for existent project

[itspec-ru]

Shotcut version: 16.11
OS: Gentoo Linux x86_64
Reproduce:

1. Run shotcut.
2. Open existent project file.
3. Try switch between source/project tabs.

Switching from "project" to "source" tab disallowed.
Output In console:
[Debug ] TimelineDock::setSelection Changing selection to (0) trackIndex -1 isMultitrack false

4. Reopen the same project file.
   Then switching from "project" to "source" tab allowed.

[ddennedy]

This is by design. At step 3, there is nothing in Source. It is only a multitrack timeline Project. The fact that you can switch between Source and Project after step 4 is because we need to close the current project to load a new one, and close tries to close everything including Source. In this case, it does not first check if Source is empty; it simply clears it with a black clip.