Mixxx complains:
Debug [Main]: WARNING: The following 6 controls were leaked: 
Debug [Main]: "[EffectRack1_EffectChain4]" "clear" ControlObject(0x2821d90) 
Debug [Main]: "[EffectRack1_EffectChain2]" "clear" ControlObject(0x201f540) 
Debug [Main]: "[EffectRack1_EffectChain3]" "clear" ControlObject(0x2424920)
I can't see the reason