Fix null reference error in addWidgetModal.js

Added checks to ensure activeForm is defined before calling its methods in the save-changes click handler, preventing potential runtime errors when activeForm is null or undefined.

modules/cms-kit/src/Volo.CmsKit.Admin.Web/Pages/CmsKit/Contents/addWidgetModal.js

@@ -25,11 +25,11 @@ $(function () {
 
             $(".save-changes").click(function () {
 
-                if(!activeForm.valid()) {
+                if (activeForm && !activeForm.valid()) {
                     return;
                 }
 
-                let properties = activeForm.serializeFormToObject();          
+                let properties = activeForm ? activeForm.serializeFormToObject() : {};          
 
                 let widgetText = "[Widget Type=\"" + widgetType + "\" ";