Execution of unstructured grid failure - Forum - Delft3D Flexible Mesh

I have created a FM grid for a section of a river with several tight bends, and flood plains connected to the main channel, as well as flood plains off the main channel and connected by small feeder channels. I created it in many pieces and then merged the various pieces together. I worked on the final, complete grid to get the orthogonality down to what I thought was quite good. The grid saves just fine (or appears to) from RGFGRID as a _net.nc file. I can import the resulting _net.nc file back into RGFGRID without problems.

However, when I try to create a project in Delft3D FM and import the grid, I get the following error:
Object reference not set to an instance of an object.

Followed by:
Execution of Unstructured Grid has failed.

I can import the individual pieces of the grid OK, but once the grid is merged into the whole, I can't isolate the problem area by cutting off pieces of the grid one at a time.

If I try to import the grid without creating a project first, just from the import menu on the home tab, then I get an unhandled exception and the program crashes.

I'm attaching a picture of the grid so you get the idea of what the system looks like.

Help in debugging this grid is greatly appreciated.