My .pri file called widgets.pri:
SOURCES += \
internal_lib/widgets/sheet_wp.cpp

HEADERS += \
internal_lib/widgets/sheet_wp.h

My .pro file has this include:
include(internal_lib/widgets/widgets.pri)

Everything was working fine but suddenly I get the following error:
error: C1083: Cannot open include file: 'internal_lib/widgets/sheet_wp.h': No such file or directory