I have a hunch that the issue reported by @bhargavy is due to a different problem. This error though is being generated in the current 5.2.4 release of ACF Pro and as soon as I reverted back to 5.2.3 it went away. So for me at least I have found a temp solution to the problem.
I have reported the issue to Elliot about this occurring in 5.2.4 and am waiting feedback.
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