Swing/AWT/SWT :: Search For A Key In Properties File - Jbutton Won't Work

I have a properties file with key values in it, now when I use my search box to search for a key in the properties file to check whether or not it is there - I can't get it to check and return the key value if it is theere. I've tried a few methods but keep getting a null pointer exception when it hits line 15 in the code all the time so I'm stumped at the minute.

My text field is called textFieldSearch

This is my button code

JButton btnSearch = new JButton("Search");
btnSearch.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
//search button
//find selected comman
String key_name = textFieldSearch.getText();

[Code] ....

This is my method code

public void FindSelectedKey()
{
if(textFieldSearch != null)
{
properties.getProperty(key_name == textFieldSearch)
}
}