Fix RTDB race condition (#9973)

Author: paulb777

FirebaseDatabase/CHANGELOG.md

@@ -1,3 +1,6 @@
+# 9.3.0
+- [fixed] Fix `reference(withPath:)` race condition crash. (#7885)
+
 # 8.12.0
 - [fixed] **Breaking change:** Mark `getData()` snapshot as nullable to fix Swift API. (#9655)
 

FirebaseDatabase/Sources/Api/FIRDatabase.m

@@ -249,10 +249,12 @@ - (void)assertUnfrozen:(NSString *)methodName {
 }
 
 - (void)ensureRepo {
-    if (self.repo == nil) {
-        self.repo = [FRepoManager createRepo:self.repoInfo
-                                      config:self.config
-                                    database:self];
+    @synchronized(self) {
+        if (self.repo == nil) {
+            self.repo = [FRepoManager createRepo:self.repoInfo
+                                          config:self.config
+                                        database:self];
+        }
     }
 }