We have a report in SSRS 2008 which calls multiple sub-reports. One of the sub-reports contains a data region (tablix) in which one of the column is populated by another sub-report i.e. the sub-report will render for each row of the tablix. This report is working fine in 2008.
As part of migration to SSRS 2008 R2, I opened the report in BIDS 2008 R2 and hit the preview. The report rendered but the tablix column contained error Error:subreport could not be shown for each row.
Hitting The View -> Error List had the error
e ERROR: An error has occurred while processing a sub-report. Details: Object reference not set to an instance of an object.
After spending nearly two days in digging, I came across this KB Article which talks about similar issue in SSRS 2008. Though this is for SSRS 2008, the work-around mentioned in the article fixed the issue. The solution is to place the main sub-report in a list control mapped to a dummy dataset with one row.