MOBILE-6262 FR9.0设计器提供手机端设计画布参考尺寸=>修复从表单切换到cpt时的npe错误

designer_form/src/com/fr/design/mainframe/FormArea.java

@@ -102,7 +102,9 @@ public class FormArea extends JComponent implements ScrollRulerComponent {
         }
         this.setFocusTraversalKeysEnabled(false);
         this.designer.addMouseWheelListener(showValSpinnerMouseWheelListener);
-        widthPane.setEnabled(!jForm.getTarget().getFormMobileAttr().isMobileOnly());
+        if (jForm != null) {
+            widthPane.setEnabled(!jForm.getTarget().getFormMobileAttr().isMobileOnly());
+        }
     }
 
     public void onMobileAttrModified() {