Create subform from a different app

Create subform from a different app

Ex Code:

Subform
(
      type=grid
      values=investigators.Investigator_Registry.ID
)

This doesn't work at all now and it was working yesterday. Now everytime I try to add something to it, it says error null (I'm paraphrasing). Also if i try to change a subform value through free-flowing script mode it gives me a error null message again. Really though I want to be able to create a subform from Investigator registry app and implant it into PCRS Project tracking app. Also I'd appreciate a more detailed message other than error null.

Ex Code:
before -

Subform
(
      type = grid
      values = test.ID
)

after-

Subform
(
      type = grid
      values = open.ID
)

This code change gives me the null error...also it doesn't matter if I have proper capitalization or not.