The JDialog spec says the following about the setLocationRelativeTo()
method:

setLocationRelativeTo
public void setLocationRelativeTo(java.awt.Component c)
    Sets the location of the dialog relative to the specified component.
    If the component is not currently showing, the dialog is centered
    on the screen.
      Parameters:
        c - the component in relation to which the dialog's location is
            determined


The spec does not specify the expected result if null is provided as
the input parameter to the setLocationRelativeTo() method.