Add comments about nullptr guard

Add some comments which describe why we need to guard agains nullptr.

src/tracks/InstrumentTrack.cpp

@@ -570,6 +570,10 @@ f_cnt_t InstrumentTrack::beatLen( NotePlayHandle * _n ) const
 
 void InstrumentTrack::playNote( NotePlayHandle* n, sampleFrame* workingBuffer )
 {
+	// Note: under certain circumstances the working buffer is a nullptr.
+	// These cases are triggered in PlayHandle::doProcessing when the play method is called with a nullptr.
+	// TODO: Find out if we can skip processing at a higher level if the buffer is nullptr.
+
 	// arpeggio- and chord-widget has to do its work -> adding sub-notes
 	// for chords/arpeggios
 	m_noteStacking.processNote( n );
@@ -580,6 +584,8 @@ void InstrumentTrack::playNote( NotePlayHandle* n, sampleFrame* workingBuffer )
 		// all is done, so now lets play the note!
 		m_instrument->playNote( n, workingBuffer );
 
+		// Calling processAudioBuffer with a nullptr leads to crashes when checking if the buffer represents silence.
+		// Therefore we must guard against a nullptr here.
 		if (workingBuffer != nullptr)
 		{
 			const fpp_t frames = n->framesLeftForCurrentPeriod();