Fix deletion of mixer channels when calling MixerView::clear (#7081)

* Fix deletion of mixer channels when calling MixerView::clear

* clear channel 0 + delete from back to front

src/gui/MixerView.cpp

@@ -590,7 +590,8 @@ void MixerView::setCurrentMixerChannel(int channel)
 
 void MixerView::clear()
 {
-	getMixer()->clear();
+	for (auto i = m_mixerChannelViews.size() - 1; i > 0; --i) { deleteChannel(i); }
+	getMixer()->clearChannel(0);
 
 	refreshDisplay();
 }