fixed menu item crash when there is no sheet opened

src/main/java/plugins/adufour/vars/gui/swing/WorkbookEditor.java

@@ -332,6 +332,9 @@ public class WorkbookEditor extends SwingVarEditor<Workbook> {
             plotHistogram1D.addMenuListener(new MenuListener() {
                 @Override
                 public void menuSelected(final MenuEvent e) {
+                    if (book.getNumberOfSheets() == 0)
+                        return;
+
                     final int sheetIndex = book.getActiveSheetIndex();
                     if (sheetIndex == -1)
                         return;
@@ -386,6 +389,9 @@ public class WorkbookEditor extends SwingVarEditor<Workbook> {
             plotHistogram2D.addMenuListener(new MenuListener() {
                 @Override
                 public void menuSelected(final MenuEvent e) {
+                    if (book.getNumberOfSheets() == 0)
+                        return;
+
                     final int sheetIndex = book.getActiveSheetIndex();
                     if (sheetIndex == -1)
                         return;
@@ -466,6 +472,9 @@ public class WorkbookEditor extends SwingVarEditor<Workbook> {
             plotScatter2D.addMenuListener(new MenuListener() {
                 @Override
                 public void menuSelected(final MenuEvent e) {
+                    if (book.getNumberOfSheets() == 0)
+                        return;
+
                     final int sheetIndex = book.getActiveSheetIndex();
                     if (sheetIndex == -1)
                         return;
@@ -541,6 +550,9 @@ public class WorkbookEditor extends SwingVarEditor<Workbook> {
             plotScatter3D.addMenuListener(new MenuListener() {
                 @Override
                 public void menuSelected(final MenuEvent e) {
+                    if (book.getNumberOfSheets() == 0)
+                        return;
+
                     final int sheetIndex = book.getActiveSheetIndex();
                     if (sheetIndex == -1)
                         return;