to me your problem looks simple....i will tell u how can u solve it..
First thing....dont a new window when you press the button. create the window
only once and
keeep that window instance as global. soo u just neeed to show the window whenn
u click the button.
hope u got wat m saying.......and null pointer exception occurs when you declare
a variable or object but you do not initialize it... and also when you are
tryinng to read  an object which has null value...that object can b a string can
b and array or anything which is null or empty.