[Solution] IntelliJ: Cannot find declaration to go to, Nothing here, Java file outside of source root Errors

If you are trying to access a class and its member variables and methods in IntelliJ IDE for an external dependency (be it Maven or Gradle) you may see a tooltip saying "Cannot find declaration to go to" if the source of the class is not available.

If you hover over your class if you see "Java file outside of source root" and when you try to run the main method you would not be able to run the application with a message "Nothing here"

Open installed Plugins (press Shift key twice) and disable Gradle and restart IntelliJ, you will be prompted to enable Gradle, once dependencies and their source are downloaded and the build is complete, this error should be fixed.