fix(Widget): throw exception when dispose (#4665)

* fix(Widget): 修复报错问题

* chore: bump version 9.0.0-beta03

Author: ArgoZhang

src/BootstrapBlazor/BootstrapBlazor.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk.Razor">
 
   <PropertyGroup>
-    <Version>9.0.0-beta02</Version>
+    <Version>9.0.0-beta03</Version>
   </PropertyGroup>
 
   <ItemGroup>

src/BootstrapBlazor/Components/DropdownWidget/DropdownWidget.razor.js

@@ -19,5 +19,9 @@ export function init(id, invoke, options) {
 }
 
 export function dispose(id) {
-    EventHandler.off(el, 'hidden.bs.dropdown');
+    const el = document.getElementById(id);
+    if (el) {
+        EventHandler.off(el, 'shown.bs.dropdown');
+        EventHandler.off(el, 'hidden.bs.dropdown');
+    }
 }