This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Can you tell us which line of code throws that exception? You'll find that information in the stack trace.
It looks like you think that line 101 of your posted code is the problem, but you catch the NumberFormatException which it might throw and print a useless message to the console. So it can't be that line. So it must be some other line. Unless perhaps you are running code which isn't the same as the code you posted.
The problem is that you are trying to manage the swapping of panels in your content pane and you are doing it incorrectly. This results in the something() method being called twice which creates two copies of your tabbed pane. One tabbed pane is added to the frame. The other just exists in memory. So the reference to the text field only exists in memory and can never be updated by the GUI and is therefore always empty.
This can be demonstrated by the following change:
Note, this is not the proper solution. You should NOT be trying to manage the swapping yourself. I am sure this is not the only problem with your code.