fix(ios): fix a crash due to race condition on RNAdMobAdHolder

ios/RNAdMobAdHolder.swift

@@ -1,19 +1,28 @@
 class RNAdMobAdHolder<T> {
+    private var adArrayQueue = DispatchQueue(label: "RNAdMobAdHolderQueue")
     private var adArray = Dictionary<Int, T>()
 
     func add(requestId: Int, ad: T) {
-        adArray.updateValue(ad, forKey: requestId)
+        adArrayQueue.sync {
+            adArray.updateValue(ad, forKey: requestId)
+        }
     }
 
     func get(requestId: Int) -> T? {
-        return adArray[requestId]
+        adArrayQueue.sync {
+            return adArray[requestId]
+        }
     }
 
     func remove(requestId: Int) {
-        adArray.removeValue(forKey: requestId)
+        adArrayQueue.sync {
+            adArray.removeValue(forKey: requestId)
+        }
     }
 
     func clear() {
-        adArray.removeAll()
+        adArrayQueue.async(flags: .barrier) {[weak self] in
+            self?.adArray.removeAll()
+        }
     }
 }