I encountered this unexpected bug while creating multiple partitions for a table in my SSAS Tabular model. While authoring the model in SSDT (Visual Studio 2010), I created two partitions for a given table to split the processing for a large dataset. After entering the query for the second partition, the validation check failed with the error “Object reference not set to an instance”.
There is a Microsoft Connect bug opened for this issue here.
You can also read more about this on the Technet thread here.