Add close() method to NewPipeDatabase

app/src/main/java/org/schabi/newpipe/NewPipeDatabase.java

@@ -51,4 +51,15 @@ public final class NewPipeDatabase {
             throw new RuntimeException("Checkpoint was blocked from completing");
         }
     }
+
+    public static void close() {
+        if (databaseInstance != null) {
+            synchronized (NewPipeDatabase.class) {
+                if (databaseInstance != null) {
+                    databaseInstance.close();
+                    databaseInstance = null;
+                }
+            }
+        }
+    }
 }

app/src/main/java/org/schabi/newpipe/util/NavigationHelper.java

@@ -609,7 +609,7 @@ public final class NavigationHelper {
      * @param activity the activity to finish
      */
     public static void restartApp(final Activity activity) {
-        NewPipeDatabase.getInstance(activity).close();
+        NewPipeDatabase.close();
         activity.finishAffinity();
         final Intent intent = new Intent(activity, MainActivity.class);
         activity.startActivity(intent);