At first glance I don't see where you assign values to clientNameText or ageValue (BTW your comment style in your declaration section makes it very difficult to read, for me anyway) before you add() them to the screen. This could be the cause of the NullPointerException.
The reason the JDE/Debugger behave the way you describe is that the exception is thrown in the MainScreen (or some super class) for which you don't have debugging data. Since the exception is not caught in your app, the only other option for the JDE is to let the Java environment catch it by terminating your app, which wouldn't help you debug.
When you see that happen, somewhere in the JDE there should be a method call history. In the list of Calling Methods should be one of yours. If you select it the source screen will show you the file/line being executed when the exception was thrown.