Debug assert when changing from 2- to 4-deck skins

[Daniel Schürmann]

When changing from Shade to Tango skin some debug assertions are violated:

```
warning [Main] ControlDoublePrivate::getControl returning NULL for ( "[Channel3]" , "end_of_track" )
critical [Main] DEBUG ASSERT: "flags.testFlag(ControlFlag::NoAssertIfMissing)" in function static QSharedPointer<ControlDoublePrivate> ControlDoublePrivate::getControl(const ConfigKey&, ControlFlags, ControlObject*, bool, bool, bool, double) at /home/daniel/workspace/2.3/src/control/control.cpp:186
critical [Main] DEBUG ASSERT: "flags & ControlFlag::AllowMissingOrInvalid" in function ControlProxy::ControlProxy(const ConfigKey&, QObject*, ControlFlags) at /home/daniel/workspace/2.3/src/control/controlproxy.cpp:16
warning [Main] ControlDoublePrivate::getControl returning NULL for ( "[Channel3]" , "time_remaining" )
critical [Main] DEBUG ASSERT: "flags.testFlag(ControlFlag::NoAssertIfMissing)" in function static QSharedPointer<ControlDoublePrivate> ControlDoublePrivate::getControl(const ConfigKey&, ControlFlags, ControlObject*, bool, bool, bool, double) at /home/daniel/workspace/2.3/src/control/control.cpp:186
critical [Main] DEBUG ASSERT: "flags & ControlFlag::AllowMissingOrInvalid" in function ControlProxy::ControlProxy(const ConfigKey&, QObject*, ControlFlags) at /home/daniel/workspace/2.3/src/control/controlproxy.cpp:16
debug [Main] WaveformWidgetFactory::setWaveformWidget - waveform widget added in factory, index 2
debug [Main] Created QGLWidget. Context Valid: true Sharing: true
warning [Main] ControlDoublePrivate::getControl returning NULL for ( "[Channel4]" , "end_of_track" )
critical [Main] DEBUG ASSERT: "flags.testFlag(ControlFlag::NoAssertIfMissing)" in function static QSharedPointer<ControlDoublePrivate> ControlDoublePrivate::getControl(const ConfigKey&, ControlFlags, ControlObject*, bool, bool, bool, double) at /home/daniel/workspace/2.3/src/control/control.cpp:186
critical [Main] DEBUG ASSERT: "flags & ControlFlag::AllowMissingOrInvalid" in function ControlProxy::ControlProxy(const ConfigKey&, QObject*, ControlFlags) at /home/daniel/workspace/2.3/src/control/controlproxy.cpp:16
warning [Main] ControlDoublePrivate::getControl returning NULL for ( "[Channel4]" , "time_remaining" )
critical [Main] DEBUG ASSERT: "flags.testFlag(ControlFlag::NoAssertIfMissing)" in function static QSharedPointer<ControlDoublePrivate> ControlDoublePrivate::getControl(const ConfigKey&, ControlFlags, ControlObject*, bool, bool, bool, double) at /home/daniel/workspace/2.3/src/control/control.cpp:186
critical [Main] DEBUG ASSERT: "flags & ControlFlag::AllowMissingOrInvalid" in function ControlProxy::ControlProxy(const ConfigKey&, QObject*, ControlFlags) at /home/daniel/workspace/2.3/src/control/controlproxy.cpp:16
```

snip

```
warning [Main] ControlDoublePrivate::getControl returning NULL for ( "[Channel3]" , "end_of_track" )
critical [Main] DEBUG ASSERT: "flags.testFlag(ControlFlag::NoAssertIfMissing)" in function static QSharedPointer<ControlDoublePrivate> ControlDoublePrivate::getControl(const ConfigKey&, ControlFlags, ControlObject*, bool, bool, bool, double) at /home/daniel/workspace/2.3/src/control/control.cpp:186
critical [Main] DEBUG ASSERT: "flags & ControlFlag::AllowMissingOrInvalid" in function ControlProxy::ControlProxy(const ConfigKey&, QObject*, ControlFlags) at /home/daniel/workspace/2.3/src/control/controlproxy.cpp:16
warning [Main] ControlDoublePrivate::getControl returning NULL for ( "[Channel3]" , "time_remaining" )
critical [Main] DEBUG ASSERT: "flags.testFlag(ControlFlag::NoAssertIfMissing)" in function static QSharedPointer<ControlDoublePrivate> ControlDoublePrivate::getControl(const ConfigKey&, ControlFlags, ControlObject*, bool, bool, bool, double) at /home/daniel/workspace/2.3/src/control/control.cpp:186
critical [Main] DEBUG ASSERT: "flags & ControlFlag::AllowMissingOrInvalid" in function ControlProxy::ControlProxy(const ConfigKey&, QObject*, ControlFlags) at /home/daniel/workspace/2.3/src/control/controlproxy.cpp:16
debug [Main] Created QGLWidget. Context Valid: true Sharing: true
warning [Main] ControlDoublePrivate::getControl returning NULL for ( "[Channel4]" , "end_of_track" )
critical [Main] DEBUG ASSERT: "flags.testFlag(ControlFlag::NoAssertIfMissing)" in function static QSharedPointer<ControlDoublePrivate> ControlDoublePrivate::getControl(const ConfigKey&, ControlFlags, ControlObject*, bool, bool, bool, double) at /home/daniel/workspace/2.3/src/control/control.cpp:186
critical [Main] DEBUG ASSERT: "flags & ControlFlag::AllowMissingOrInvalid" in function ControlProxy::ControlProxy(const ConfigKey&, QObject*, ControlFlags) at /home/daniel/workspace/2.3/src/control/controlproxy.cpp:16
warning [Main] ControlDoublePrivate::getControl returning NULL for ( "[Channel4]" , "time_remaining" )
critical [Main] DEBUG ASSERT: "flags.testFlag(ControlFlag::NoAssertIfMissing)" in function static QSharedPointer<ControlDoublePrivate> ControlDoublePrivate::getControl(const ConfigKey&, ControlFlags, ControlObject*, bool, bool, bool, double) at /home/daniel/workspace/2.3/src/control/control.cpp:186
critical [Main] DEBUG ASSERT: "flags & ControlFlag::AllowMissingOrInvalid" in function ControlProxy::ControlProxy(const ConfigKey&, QObject*, ControlFlags) at /home/daniel/workspace/2.3/src/control/controlproxy.cpp:16
```

[ronso0 (ronso0)]

I'm pretty sure it happens when switching to any 4-deck skin.
See https://mixxx.zulipchat.com/#narrow/stream/247620-development-help/topic/qDebug.20messages.20with.20DEBUG_ASSERTIONS_FATAL

IF it happened, it did for various COs, all of Channel3

[ronso0 (ronso0)]

https://github.com/mixxxdj/mixxx/pull/4421

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/10558