Linux: ensure that old LinuxPopupMenuCanceler window listener is removed before adding a new one (issue #962)

(did not yet happen...)

flatlaf-core/src/main/java/com/formdev/flatlaf/LinuxPopupMenuCanceler.java

@@ -70,6 +70,8 @@ class LinuxPopupMenuCanceler
 	}
 
 	private void addWindowListeners( MenuElement selected ) {
+		removeWindowListeners();
+
 		// see BasicPopupMenuUI.MouseGrabber.grabWindow()
 		Component invoker = selected.getComponent();
 		if( invoker instanceof JPopupMenu )