nullify context in onDetachedFromEngine

Author: jaredmixpanel

android/src/main/java/com/mixpanel/mixpanel_flutter/MixpanelFlutterPlugin.java

@@ -531,5 +531,6 @@ public void onDetachedFromEngine(@NonNull FlutterPluginBinding binding) {
             channel = null;
         }
         flutterPluginBinding = null;
+        context = null;
     }
 }