Right click on project -> add new... -> Qt -> Qt Designer Form Class -> Choose... -> Template Widget -> Change classname to anything but "Form"

Result in 5.4.1:
cpp:

test::test(QWidget *parent) :
QWidget(parent),
ui(new Ui::Form)

h:

private:
Ui::Form *ui;

Result in 5.4.0:
cpp:

test::test(QWidget *parent) :
QWidget(parent),
ui(new Ui::test)

h:

private:
Ui::test *ui;

5.4.1 sets wrong variable names which results in "invalid use of incomplete type..."