you are triying add container in this container. Problem is here
Container mainContainer = mainFrame.getContentPane();
...
mainFrame.add(mainContainer);
this JFrame mainFrame = this.getMainFrame();
already have ContentPane
, and you adding one more. Each JFrame have 3 panels -> glassPane, contentPane and layerdPane.
Try add elements in mainContainer
(that already included in mainFrame
).