Comment 2 for bug 1971422

I tried to add a new color type and two variables of this new color type. Then remove the second one and I get the following:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException: Cannot invoke "Object.equals(Object)" because the return value of "javax.swing.JList.getSelectedValue()" is null
        at net.tapaal.gui.petrinet.editor.ConstantsPane.enableButtons(ConstantsPane.java:455)
        at net.tapaal.gui.petrinet.editor.ConstantsPane$VariablesListModel.removeElement(ConstantsPane.java:807)
        at net.tapaal.gui.petrinet.editor.ConstantsPane.removeVariables(ConstantsPane.java:697)
        at net.tapaal.gui.petrinet.editor.ConstantsPane.lambda$addConstantsButtons$2(ConstantsPane.java:399)