Fixed addButton on click exception

[chqu1012]

	addButton.addSelectionListener(new SelectionAdapter() {
		override void widgetSelected(SelectionEvent e) {
			var ISelection selection = editableAttributesListViewer.getSelection()
			if (selection instanceof IStructuredSelection) {
				var IStructuredSelection ss = (selection as IStructuredSelection)
				val method = ss.firstElement as IMethod
				usedAttributes+=method
				availableAttributes-=method
				editableAttributesListViewer.refresh()
				usedAttributeListViewer.refresh()
			}
		}
	})