Show message instead crash

Signed-off-by: alperozturk <alper_ozturk@proton.me>

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

@@ -899,6 +899,10 @@ public class ReceiveExternalFilesActivity extends FileActivity
     }
 
     public void uploadFiles() {
+        if (mStreamsToUpload == null) {
+            DisplayUtils.showSnackMessage(this, R.string.receive_external_files_activity_unable_to_find_file_to_upload);
+            return;
+        }
 
         UriUploader uploader = new UriUploader(
             this,

app/src/main/res/values/strings.xml

@@ -452,6 +452,7 @@
     <string name="clipboard_label">Text copied from %1$s</string>
 
     <string name="receive_external_files_activity_start_sync_folder_is_not_exists_message">Folder cannot be found, sync operation is cancelled</string>
+    <string name="receive_external_files_activity_unable_to_find_file_to_upload">Unable to find file to upload</string>
 
     <string name="error_cant_bind_to_operations_service">Critical error: Unable to perform operations</string>