After the _finishCallback for the CollectionCloner is called, the cloner may be destroyed, so the code must avoid touching method variables after calling it. This is violated by the exit guard for _runQuery, since _runQuery can call _finishCallback.

We should be able to avoid this by creating the onCompletionGuard now created in _runQuery in the lambda function which calls _runQuery, then using it instead of _finishCallback. The _finishCallback can be run in the guard after the current operations run in ON_BLOCK_EXIT, and the ON_BLOCK_EXIT can be removed.