From release 30 ADRIFT began throwing exceptions when you try to run several tasks in a row using actions.
A simple TAF file showing the problem can be found at:
It has 4 tasks that can be run individually with no problems.
If run using actions from another task then it gets the error "Object reference not set to an instance of an object" about when the 4th task tries to output its text box.
pinosante has an example with the same problem here:
and AStrauss here: