Move all files to natural position.

app/src/main/java/org/thoughtcrime/securesms/MainFragment.java

@@ -0,0 +1,22 @@
+package org.thoughtcrime.securesms;
+
+import android.content.Context;
+
+import androidx.annotation.NonNull;
+import androidx.fragment.app.Fragment;
+
+public class MainFragment extends Fragment {
+
+  @Override
+  public void onAttach(@NonNull Context context) {
+    super.onAttach(context);
+
+    if (!(requireActivity() instanceof MainActivity)) {
+      throw new IllegalStateException("Can only be used inside of MainActivity!");
+    }
+  }
+
+  protected @NonNull MainNavigator getNavigator() {
+    return MainNavigator.get(requireActivity());
+  }
+}