Add some sanity check on the URL parameter

@@ -69,6 +69,12 @@
         url = [NSURL URLWithString:(NSString *)url];
     }
 
+    // Prevents app crashing on argument type error like sending NSNull instead of NSURL
+    if (![url isKindOfClass:NSURL.class])
+    {
+        url = nil;
+    }
+
     __block SDWebImageCombinedOperation *operation = SDWebImageCombinedOperation.new;
     
     if (!url || !completedBlock || (!(options & SDWebImageRetryFailed) && [self.failedURLs containsObject:url]))