GUI text editor bug.

git-svn-id: https://www.prismmodelchecker.org/svn/prism/prism/trunk@670 bbc10eb1-c90d-0410-af57-cb519fbb1720

prism/src/userinterface/model/GUITextModelEditor.java

@@ -703,6 +703,8 @@ public class GUITextModelEditor extends GUIModelEditor implements DocumentListen
 	
 	public int computeDocumentOffset(int line, int column) throws BadLocationException
 	{
+		if (line < 0 || column < 0) throw new BadLocationException("Negative line/col", -1);
+		
 		Element lineElement = editor.getDocument().getDefaultRootElement().
 		getElement(line-1);