Object Reference not set to an instance of an object error message after running initial condition and then validating the model.
There has been at least one case where this was caused by missing pipe start or stop nodes after running Skelebrator. Deleting those elements resolved the problem.
1) Open the pipe flextable and sort the Start Node column in ascending order
2) Left click and hold then drag the cursor down the row headers until all the columns with no start node are selected. All these elements are inactive and should be highlighted in blue.
3) Right click on one of those selected columns and choose 'Open on Selection'. This will filter the table to only those rows. Note that you will need 08.11.06.58 or higher to see this feature.
4) Click the dropdown arrow next to the second icon from the right that looks like a little chart. Choose 'Create a selection set' and name the selection set.
5) Go to View > Selection Sets and locate that selection set. Double click on it to highlight those elements and then click delete. Zoom in and examine first if needed.
6) Repeat steps 1-5 for the stop node column.
7) Save a copy of the model, re-compute initial conditions and validate.