fix: guard startSyncFolderOperation against a null folder (#4738)

changelog/unreleased/4738

@@ -0,0 +1,10 @@
+Bugfix: Prevent NullPointerException when returning from external-file receive screen
+
+The share-into flow's `ReceiveExternalFilesActivity` forwarded a folder to
+`ReceiveExternalFilesViewModel.refreshFolderUseCase` without guarding for null,
+so any code path where `getCurrentDir()` resolved to `null` — such as leaving
+the screen before the storage manager had loaded the parent folder — crashed
+with a Kotlin platform-type NPE. The activity now skips the sync when the
+folder cannot be resolved and logs the reason instead of crashing.
+
+https://github.com/owncloud/android/issues/4738

owncloudApp/src/main/java/com/owncloud/android/ui/activity/ReceiveExternalFilesActivity.java

@@ -628,6 +628,14 @@ public void onSavedCertificate() {
     }
 
     private void startSyncFolderOperation(OCFile folder) {
+        // getCurrentDir() and other callers can hand us a null folder when the
+        // storage manager cannot resolve the parent path yet; forwarding that
+        // to the Kotlin use case would trip the platform-type null check and
+        // crash as reported in issue #4738.
+        if (folder == null) {
+            Timber.w("startSyncFolderOperation called with a null folder; skipping sync");
+            return;
+        }
 
         mSyncInProgress = true;