feat(Watermark): add data-bb-watermark attribute (#5825)

* refactor: 增加全局水印时移动到文档根节点逻辑

* refactor: 增加变量保护

* refactor: 完善 dispose 逻辑防止 dom 残留

Author: ArgoZhang

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

@@ -41,10 +41,12 @@ export function dispose(id) {
     Data.remove(id);
 
     if (watermark) {
-        const { ob } = watermark;
+        const { el, ob } = watermark;
         ob.disconnect();
 
         delete watermark.ob;
+        document.body.removeAttribute('data-bb-watermark');
+        el.remove();
     }
 }
 
@@ -102,13 +104,13 @@ const createWatermark = watermark => {
     if (mark) {
         mark.remove();
     }
+    el.appendChild(div);
 
-    if (bg.isPage) {
-        document.body.appendChild(div);
-    }
-    else {
-        el.appendChild(div);
+    if (options.isPage) {
+        document.body.setAttribute('data-bb-watermark', "true");
+        document.body.appendChild(el);
     }
+
     options.bg = bg;
     requestAnimationFrame(() => monitor(watermark));
 }
@@ -179,8 +181,12 @@ const monitor = watermark => {
 
 const clearWatermark = watermark => {
     const { el, ob } = watermark;
-    ob.disconnect();
-    el.innerHTML = '';
+    if (ob) {
+        ob.disconnect();
+    }
+    if (el) {
+        el.innerHTML = '';
+    }
 }
 
 const getWatermark = props => {