Let GC take care of the presenter. Also fix #947

app/src/main/AndroidManifest.xml

@@ -21,7 +21,7 @@
         android:theme="@style/Theme.Tachiyomi">
         <activity
             android:name=".ui.main.MainActivity"
-            android:launchMode="singleTask">
+            android:launchMode="singleTop">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
 

app/src/main/java/eu/kanade/tachiyomi/ui/base/presenter/NucleusConductorDelegate.java

@@ -61,7 +61,6 @@ public class NucleusConductorDelegate<P extends Presenter> {
     void onDestroy() {
         if (presenter != null) {
             presenter.destroy();
-            presenter = null;
         }
     }
 }