No documents matching ui_mainwindow.h could be found error

Using Qt creator with an existing project, project has 2 .h files, 2 .cpp files and 2 .ui files - the file I am attempting to edit is "mainwindow.ui" and when trying to add a slot I am getting the error "No documents matching ui_mainwindow.h could be found error. Rebuilding the project may help". Have tried rebuilding, cleaning, clearing Qt files and re-opening the project, nothing is resolving the issue. ui_mainwindow.h exists in the build directory only as expected, the project can build fine. When editing the other .ui file I get no such error and all works fine.