[GREENFOOT-57] NPE when creating new subclass

teps to reproduce: \- rename one of the existing Actor classes with the editor and save the class \- create a new subclass of an Actor class

This error will be printed: Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at greenfoot.gui.classbrowser.ClassBrowser.createClassHierarchyComponent(ClassBrowser.java:220) at greenfoot.gui.classbrowser.ClassBrowser.createClassHierarchyComponent(ClassBrowser.java:233) at greenfoot.gui.classbrowser.ClassBrowser.updateLayout(ClassBrowser.java:135) at greenfoot.gui.classbrowser.ClassBrowser.addClass(ClassBrowser.java:84) at ...

Issue metadata

Issue type: Bug
Priority: Low
Fix versions: 1.4.1