Fix crash in RCTImagePicker on iOS

Summary:
@public
Apparently if you select images really quickly, the app will crash.

Reviewed By: shergin

Differential Revision: D8796730

fbshipit-source-id: 81a652d9c8997ee9e83bbee3378954a6718a9b5d

Libraries/CameraRoll/RCTImagePickerManager.m

@@ -153,6 +153,11 @@ didFinishPickingMediaWithInfo:(NSDictionary<NSString *, id> *)info
 - (void)_dismissPicker:(UIImagePickerController *)picker args:(NSArray *)args
 {
   NSUInteger index = [_pickers indexOfObject:picker];
+  if (index == NSNotFound) {
+    // This happens if the user selects multiple items in succession.
+    return;
+  }
+
   RCTResponseSenderBlock successCallback = _pickerCallbacks[index];
   RCTResponseSenderBlock cancelCallback = _pickerCancelCallbacks[index];