feat(Watermark): update logic to prevent errors (#5791)

* refactor: 修复报错

* refactor: 更新 z-index 值

Author: ArgoZhang

src/BootstrapBlazor/Components/Watermark/Watermark.razor.js

@@ -51,7 +51,7 @@ export function dispose(id) {
 const updateWatermark = (records, watermark) => {
     for (const record of records) {
         for (const dom of record.removedNodes) {
-            if (dom.classList.contains('bb-watermark-bg')) {
+            if (dom.classList && dom.classList.contains('bb-watermark-bg')) {
                 createWatermark(watermark);
                 return;
             }
@@ -90,7 +90,7 @@ const createWatermark = watermark => {
     div.style.opacity = '1';
     div.style.position = 'absolute';
     div.style.inset = '0';
-    div.style.zIndex = '999';
+    div.style.zIndex = '9999';
     div.classList.add("bb-watermark-bg");
 
     const mark = el.querySelector('.bb-watermark-bg');
@@ -138,7 +138,7 @@ const monitor = watermark => {
         clearWatermark(watermark);
         return;
     }
-    if (zIndex !== '999') {
+    if (zIndex !== '9999') {
         clearWatermark(watermark);
         return;
     }