Fix audio visualizer exception (#1008)

Fix `MissingPluginException` when stopping audio visualizer.

.changes/fix-audio-visualizer-missing-plugin-exception

@@ -0,0 +1 @@
+patch type="fixed" "MissingPluginException when stopping audio visualizer on Android/iOS"

lib/src/track/audio_visualizer_native.dart

@@ -53,11 +53,13 @@ class AudioVisualizerNative extends AudioVisualizer {
       return;
     }
 
-    await Native.stopVisualizer(mediaStreamTrack.id!, visualizerId: visualizerId);
-
+    // Cancel subscription before native stop, otherwise the native
+    // StreamHandler is already removed and cancel throws MissingPluginException.
     await _streamSubscription?.cancel();
     _streamSubscription = null;
     _eventChannel = null;
+
+    await Native.stopVisualizer(mediaStreamTrack.id!, visualizerId: visualizerId);
   }
 }
 

shared_swift/LiveKitPlugin.swift

@@ -201,6 +201,7 @@ public class LiveKitPlugin: NSObject, FlutterPlugin {
         }
 
         for processors in audioProcessors.values {
+            processors.visualizers[visualizerId]?.stop()
             processors.visualizers.removeValue(forKey: visualizerId)
         }
 

shared_swift/Visualizer.swift

@@ -69,8 +69,13 @@ public class Visualizer: NSObject, RTCAudioRenderer, FlutterStreamHandler {
         channel?.setStreamHandler(self)
     }
 
-    deinit {
+    public func stop() {
         _track?.remove(audioRenderer: self)
+        channel?.setStreamHandler(nil)
+    }
+
+    deinit {
+        stop()
     }
 
     public func render(pcmBuffer: AVAudioPCMBuffer) {