Grails: Cannot do code navigation when missing webapp folder

What steps will reproduce the issue?
1. Create a grails 3 project, and remove the src/main/webapp folder
2. Import the project with Intellij, you will see that you **cannot** navigate code with Cmd+B.

_⚠️ attachment `2018-04-28 10.59.png` was not included in the export_

What is the expected result?
Intellij should import grails project even though the project is missing the src/main/webapp folder, because many people like me use grails for headless restful web services.

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*1 attachment(s) referenced here (`2018-04-28 10.59.png`) are not part of the CSV export.*
*Imported from [IDEA-191161](https://youtrack.jetbrains.com/issue/IDEA-191161) · Type: Usability Problem · Votes: 0*
*Comments, attachments, dates and reporter are not part of the export — follow the link above for the full history.*

Reproduce the issue with a Grails 3 project after removing src/main/webapp, then import it into IntelliJ and test Cmd+B code navigation. Trace the project import behavior related to the missing folder. Done means the project imports successfully and code navigation works for a headless REST service project without src/main/webapp.

Written by the indexing model from the issue text.