Yes I've seen it reported on this forum.
Search this error on Google.
https://www.google.com/search?ei=26yuWs6LCdGzzwKrlbHoAQ&q=attempt+to+index+a+nil+value+lig htroom&oq=attempt+to+index+a+nil+value+lightroom&gs_l=psy-ab.12...0.0.0.113184.108.40.206.0.0.0. 0.0..0.0....0...1c..64.psy-ab..0.0.0....0.LA6GlyPTzEg
Thank you for your answer. This error message goes back to LR2 and LR3. Unfortunately I have not come up with a conclusive solution to my problem.
Note that this error message doesn't have a single cause. "Attempt to index nil value" is a low-level programming error that occurs whenever the program tries to reference a nil (empty) value as if it were a non-nil object. It's one of the most common programming errors in Lua (the language used to implement LR).
So while it can be helpful to search for previously reported instances of the error message, beware that it arises from many different bugs inside LR, and a solution for one occurrence of the message probably won't help with other occurrences.
Thank you for you comment John, I agree with you, the sources of the error seem to vary a lot. At the moment this only occurs once LR is fired up and everything works well. I will not attempt anything if I am not sure it will be targeted towards my specific case.