Fix a crash with back button

app/src/main/java/app/fedilab/android/mastodon/helper/Helper.java

@@ -909,7 +909,10 @@ public class Helper {
             if (args != null) fragment.setArguments(args);
             ft = fragmentManager.beginTransaction();
             ft.add(containerViewId, fragment, tag);
-            if (backStackName != null) ft.addToBackStack(backStackName);
+            if (backStackName != null)  {
+                ft = fragmentManager.beginTransaction();
+                ft.addToBackStack(backStackName);
+            }
             if (!fragmentManager.isDestroyed()) {
                 ft.commit();
             }

app/src/main/java/app/fedilab/android/mastodon/ui/fragment/timeline/FragmentMastodonTimeline.java

@@ -1259,7 +1259,7 @@ public class FragmentMastodonTimeline extends Fragment implements StatusAdapter.
             if (show_pinned) {
                 //Fetch pinned statuses to display them at the top
                 accountsVM.getAccountStatuses(currentInstance, MainActivity.currentToken, accountId, null, null, null, null, null, false, true, MastodonHelper.statusesPerCall(requireActivity()))
-                        .observe(getViewLifecycleOwner(), pinnedStatuses -> accountsVM.getAccountStatuses(BaseMainActivity.currentInstance, BaseMainActivity.currentToken, accountTimeline.id, null, null, null, exclude_replies, exclude_reblogs, media_only, false, MastodonHelper.statusesPerCall(requireActivity()))
+                        .observe(getViewLifecycleOwner(), pinnedStatuses -> accountsVM.getAccountStatuses(BaseMainActivity.currentInstance, BaseMainActivity.currentToken, accountId, null, null, null, exclude_replies, exclude_reblogs, media_only, false, MastodonHelper.statusesPerCall(requireActivity()))
                                 .observe(getViewLifecycleOwner(), otherStatuses -> {
                                     if (otherStatuses != null && otherStatuses.statuses != null && pinnedStatuses != null && pinnedStatuses.statuses != null) {
                                         for (Status status : pinnedStatuses.statuses) {