fix crash enabling PFL after an EffectState is deleted

src/engine/effects/engineeffect.cpp

@@ -72,6 +72,17 @@ void EngineEffect::loadStatesForInputChannel(const ChannelHandle* inputChannel,
 // Called from the main thread for garbage collection after an input channel is disabled
 void EngineEffect::deleteStatesForInputChannel(const ChannelHandle* inputChannel) {
     m_pProcessor->deleteStatesForInputChannel(inputChannel);
+    // If an output channel is not presently being processed, for example when
+    // PFL is not active, then EngineEffect::process cannot be relied upon to
+    // set the EffectEnableState from Disabling to Disabled. This must be done
+    // before the next time EngineEffect::process is called for that output
+    // channel, otherwise the EffectEnableState will still be in the intermediate
+    // Disabling state and EffectProcessorImpl::processChannel will try to run
+    // with an EffectState that has already been deleted and cause a crash.
+    // Refer to https://bugs.launchpad.net/mixxx/+bug/1741213
+    for (EffectEnableState& effectState : m_effectEnableStateForChannelMatrix[*inputChannel]) {
+        effectState = EffectEnableState::Disabled;
+    }
 }
 
 bool EngineEffect::processEffectsRequest(EffectsRequest& message,