fix(android, utils): fix rare crash getting documents directory (#5118)

* Fix: value for key KEY_DOCUMENT_DIRECTORY not exists if Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT and externalDirectory != null

* fix(android, utils): fix rare crash getting documents directory

this ensures the documents directory always exists, at the expense
of having it be a different value under rare circumstances (better
than crashing at least...)

Docs updated to reflect the rare cases it can return unexpected values

Co-authored-by: Mike Hardy <github@mikehardy.net>

packages/app/android/src/reactnative/java/io/invertase/firebase/utils/ReactNativeFirebaseUtilsModule.java

@@ -161,10 +161,18 @@ public Map<String, Object> getConstants() {
 
     File externalDirectory = context.getExternalFilesDir(null);
     if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
-      if ( externalDirectory != null ) {
+      if (externalDirectory != null) {
         constants.put(KEY_DOCUMENT_DIRECTORY, externalDirectory.getAbsolutePath());
+      } else {
+        // The external directory may be null if it is truly external *and*
+        // the device's external storage environment changes. We will use the regular
+        // Files directory as a backup and note in the documentation that the directory may
+        // vary under rare conditions
+        constants.put(KEY_DOCUMENT_DIRECTORY, context.getFilesDir().getAbsolutePath());
       }
-    } else {
+    }
+
+    if (!constants.containsKey(KEY_DOCUMENT_DIRECTORY))  {
       constants.put(KEY_DOCUMENT_DIRECTORY, context.getFilesDir().getAbsolutePath());
     }
 

packages/app/lib/index.d.ts

@@ -284,6 +284,11 @@ export namespace Utils {
      *
      * Use this directory to place documents that have been created by the user.
      *
+     * Normally this is the external files directory on Android but if no external storage directory found,
+     * e.g. removable media has been ejected by the user, it will fall back to internal storage. This may
+     * under rare circumstances where device storage environment changes cause the directory to be different
+     * between runs of the application
+     *
      * ```js
      * firebase.utils.FilePath.DOCUMENT_DIRECTORY;
      * ```