Mark cannot cast as an error message properly.

src/org/leumasjaffe/charsheet/view/magic/SpellMenu.java

@@ -34,7 +34,7 @@ class SpellMenu extends JPopupMenu {
 			mntmCast.addActionListener(e -> {
 				final DDSpellbook book = dclass.getSpellBook().get();
 				if (book.numSpellsPerDayRemainingAtLevel(spellLevel) == 0) {
-					JOptionPane.showMessageDialog(null, "Cannot cast any more spells");
+					JOptionPane.showMessageDialog(this, "Cannot cast any more spells", "Error", JOptionPane.ERROR_MESSAGE);
 					return;
 				}
 				book.castSpell(spellLevel, spell);