Fixed null pointer exception by properly replacing the fragment

app/src/main/java/acr/browser/lightning/activity/BrowserActivity.java

@@ -316,8 +316,8 @@ public abstract class BrowserActivity extends ThemableBrowserActivity implements
         final FragmentManager fragmentManager = getSupportFragmentManager();
         fragmentManager
                 .beginTransaction()
-                .add(containerId, tabsFragment)
-                .add(R.id.right_drawer, bookmarksFragment)
+                .replace(containerId, tabsFragment)
+                .replace(R.id.right_drawer, bookmarksFragment)
                 .commit();
         if (mShowTabsInDrawer) {
             mToolbarLayout.removeView(findViewById(R.id.tabs_toolbar_container));

app/src/main/java/acr/browser/lightning/fragment/TabsFragment.java

@@ -243,7 +243,7 @@ public class TabsFragment extends Fragment implements View.OnClickListener, View
         }
     }
 
-    public class LightningViewAdapter extends RecyclerView.Adapter<LightningViewAdapter.LightningViewHolder> {
+    private class LightningViewAdapter extends RecyclerView.Adapter<LightningViewAdapter.LightningViewHolder> {
 
         private final int mLayoutResourceId;
         @Nullable private final Drawable mBackgroundTabDrawable;