Disable the debugger thread selector when there's nothing to select

[passivestar]

A small UI fix. Empty lists just have this tiny rectangle appear when there's nothing in them and it looks broken

Old	New

[Geometror]

When starting the project it's still active and empty until you reach a breakpoint/pause the project. You should probably check for the thread count instead of the active flag.

[Repiteo]

Thanks!