Prohibit trigger of note on instrument-less FM channel

src/reskit/soundtrack/engines/echo/engine.rs

@@ -312,7 +312,18 @@ pub fn get_events_for_channel( channels: &mut [Channel], active_channel: usize,
             channels[ active_channel ].note_keyed = false;
             events.push( vec![ ESF_NOTE_OFF | channels[ active_channel ].id ] );
         } else {
-            channels[ active_channel ].note_keyed = true;
+            // Determine if we can key on. We can't key on if we are in FM1..FM6, and no instrument was ever set
+            // on the channel either here or previously. (We can do that for psg, using the default instrument.)
+            channels[ active_channel ].note_keyed = if ( ESF_FM_1..=ESF_FM_3 ).contains( &channels[ active_channel ].id ) | ( ESF_FM_4..=ESF_FM_6 ).contains( &channels[ active_channel ].id ) {
+                if channels[ active_channel ].active_instrument.is_none() {
+                    print_warning( "no instrument set on this fm channel - cannot trigger the note!" );
+                    false
+                } else {
+                    true
+                }
+            } else {
+                true
+            };
             channels[ active_channel ].active_note = Some(
                 OctaveFrequency {
                     octave: note.get_octave()?,