fix: Crash when last upload folder does not exist anymore

- When browsing back from a folder, directly browse to root if parent does not exist
- When initially opening a folder, directly browse to root if it doesn't exist

Fixes #11207

Signed-off-by: Álvaro Brey <alvaro.brey@nextcloud.com>

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

@@ -257,7 +257,14 @@ public class ReceiveExternalFilesActivity extends FileActivity
         }
 
         initTargetFolder();
+        String full_path = generatePath(mParents);
+        final OCFile fileByPath = getStorageManager().getFileByPath(full_path);
+        if (fileByPath != null) {
+            startSyncFolderOperation(fileByPath);
             populateDirectoryList();
+        } else {
+            browseToRoot();
+        }
     }
 
     @Override
@@ -623,8 +630,13 @@ public class ReceiveExternalFilesActivity extends FileActivity
         } else {
             mParents.pop();
             String full_path = generatePath(mParents);
-            startSyncFolderOperation(getStorageManager().getFileByPath(full_path));
+            final OCFile fileByPath = getStorageManager().getFileByPath(full_path);
+            if (fileByPath != null) {
+                startSyncFolderOperation(fileByPath);
                 populateDirectoryList();
+            } else {
+                browseToRoot();
+            }
         }
     }
 
@@ -826,6 +838,10 @@ public class ReceiveExternalFilesActivity extends FileActivity
     }
 
     private void startSyncFolderOperation(OCFile folder) {
+        if (folder == null) {
+            throw new IllegalArgumentException("Folder must not be null");
+        }
+
         long currentSyncTime = System.currentTimeMillis();
 
         mSyncInProgress = true;
@@ -1078,6 +1094,8 @@ public class ReceiveExternalFilesActivity extends FileActivity
     private void browseToRoot() {
         OCFile root = getStorageManager().getFileByPath(OCFile.ROOT_PATH);
         mFile = root;
+        mParents.clear();
+        mParents.add("");
         startSyncFolderOperation(root);
     }