UI designer form broken and does not work anymore

After some manipulations, I have lost the connection between form file and it's bound class.

The following code

public static void main(String[] args) {
   JFrame frame = new JFrame("MyForm");
   JPanel mainPanel = new Test03().mainPanel;
   frame.setContentPane(mainPanel);
   frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
   frame.pack();
   frame.setVisible(true);
}

does not work, having mainPanel null as it should be since it is not initialized. 

Resetting "bind to class" didn't help.

Setting GUI Designer / Generate GUI Into = Java source code didn't help. Some code appeared in the class, but inspection says classes like 

com.intellij.uiDesigner.core.GridConstraints

are not found. No any way to autofix error and/or import some jars. No any known jar name for uiDesigner known and/or documented.

What to do?