Move to index 0 if previousChildKey is not found

Author: ramblex

FirebaseDatabaseUI/FirebaseArray.m

@@ -123,7 +123,11 @@ - (void)initListeners {
         NSUInteger fromIndex = [self indexForKey:snapshot.key];
         [self.snapshots removeObjectAtIndex:fromIndex];
 
-        NSUInteger toIndex = [self indexForKey:previousChildKey] + 1;
+        NSUInteger toIndex = 0;
+        NSUInteger prevIndex = [self indexForKey:previousChildKey];
+        if (prevIndex != NSNotFound) {
+          toIndex = prevIndex + 1;
+        }
         [self.snapshots insertObject:snapshot atIndex:toIndex];
 
         if ([self.delegate respondsToSelector:@selector(array:didMoveObject:fromIndex:toIndex:)]) {