Java Web Start getting two Windows when only need one

Hello I am migrating my Java applet to Java Web Start as Firefox disabled Java NPAPI support.

I am using JNLP to open to load the applet. Inside the applet I am extending Applet Class to the main class.

When we open the applet a File Chooser is opened where we have to select a folder and save its path.

But when we launch web start application I get 2 windows one window is empty and another window of File Chooser which I opened using JFileChooser.showSaveDialog.

I want the File Chooser to open in the main window started by web start and not in a new window. Please tell me how to do it.