Unhandled exception: object reference not set to an instance of an object

[jbscherrer]

I'm writing a workflow that takes in streaming video, analyzes activity in a few ROIs, and controls a digital output based on the activity. A first-pass is working fine:

But now let's say I want to update the ROI coordinates frame-by-frame based on a motion correction signal. My understanding is that I then have to put the RoiActivity node in a SelectMany, so that RoiActivity processes each frame together with its corresponding coordinates. At first, things seem to be working ok when I do this, and RoiActivity continues to output a stream of activity values.

However, when I try to open the RoiActivity visualizer, I get an unhandled exception error: Object reference not set to an instance of an object. Occasionally I will also get an error: Failed to make context XXX current as well.

Any idea what may be happening? I'm attaching the workflow if that is helpful.

roi_error.zip

[jbscherrer]

I realized I could get RoiActivity to synchronously update its coordinates with each frame by zipping both together and then unzipping into RoiActivity, so this issue is no longer pressing. If anyone knows why this error occurs though, I would be curious to know.