Solution provided by Mathworks Tech Support. File not attached to this answer, so get from Tech support.
This appears to be an issue in R2018a which is addressed by following the steps below.
1) Download the 'createDialogForMACheck.p' file that is attached to this email.
2) Navigate to the following folder: (matlabroot)\toolbox\simulink\simulink\modeladvisor\+ModelAdvisor\@Task
3) Rename the existing createDialogForMACheck.p to createDialogForMACheck_backup.p
4) Copy the attached createDialogForMACheck.p to this folder.
5). Execute the following command in the MATLAB command window: >> rehash toolboxreset
6) Restart MATLAB.