I believe this problem is either a bug or corrupted ism file.

Although IS does not store data in SQL database, trying to apply similar concepts, you would agree with me that the program is trying to save a .null. value into a field who does not accept such values; since you can't modify that in IS Express for VFP, the only solution seems to be to fill that entry with some string value.

1. Open your ism project
2. Click on plus sign to open "Organize your setup” option branch
3. Look for every single entry that requires a text entry and make sure there’s a string entered for each one, if not, enter some text, example being, “Publisher/Product URL” needs a string value, and so on.
4. Rebuild your project, be happy again. If still did not work look into the rest of the project where a string entry is required and do your thing.

It is difficult, if not impossible, with this limited version of IS to determine the offending field by using the description given in the error message  'ID_STRING1'.

You could easily know this however. That is in the case you had the full version of IS Express:

• Use the menu option "Project-Export String Table”  to create a text file with your project info. Do this at least once before you build the project the first time.
• When you get an error, open this text file and look the value reported in the error message to which project option refers
• Go back to your ise file and correct only the offending field.

This is how the txt file looks like: