!3225 fix(#I5QH52): throw exception when not set UploaderElement in sub class of UploadBase

* fix: 修复子类未设置 UploaderElement 导致脚本报错问题

Author: ArgoZhang

src/BootstrapBlazor/Components/Upload/UploadBase.cs

@@ -69,7 +69,7 @@ protected override async Task OnAfterRenderAsync(bool firstRender)
     {
         await base.OnAfterRenderAsync(firstRender);
 
-        if (firstRender && !IsDisabled)
+        if (firstRender && !IsDisabled && UploaderElement.Context != null)
         {
             // support drag
             Module = await JSRuntime.LoadModule("upload.js");
@@ -169,7 +169,7 @@ protected virtual IDictionary<string, object> GetUploadAdditionalAttributes()
         if (!string.IsNullOrEmpty(Capture))
         {
             ret.Add("capture", Capture);
-        }        
+        }
         return ret;
     }