MudElement: Fix ElementReferenceCapture (MudBlazor#9547)

Co-authored-by: Artyom M. <wordkap@gmail.com>

src/MudBlazor.UnitTests.Viewer/TestComponents/Element/ElementReferenceExceptionTest.razor

@@ -0,0 +1,20 @@
+@namespace MudBlazor.UnitTests.TestComponents
+
+@* https://github.com/MudBlazor/MudBlazor/issues/9544 *@
+
+<MudButton id="element-button"
+           Variant="@Variant.Filled"
+           Color="@Color.Primary"
+           OnClick="@ButtonOnClick"
+           tabindex="@_tabindex">
+    Error
+</MudButton>
+
+@code {
+    private string _tabindex = "-1";
+
+    void ButtonOnClick()
+    {
+        _tabindex = null;
+    }
+}

src/MudBlazor.UnitTests/Components/ElementTests.cs

@@ -48,5 +48,12 @@ public void MudElement_Should_Not_Attach_A_Null_Event()
             comp2.Find("span").MouseOver();
             comp2.Instance.WasHovered.Should().BeTrue();
         }
+
+        [Test]
+        public void ElementReferenceCapture()
+        {
+            var comp = Context.RenderComponent<ElementReferenceExceptionTest>();
+            comp.Find("#element-button").Click();
+        }
     }
 }

src/MudBlazor/Components/Element/MudElement.cs

@@ -75,16 +75,7 @@ protected override void BuildRenderTree(RenderTreeBuilder builder)
             builder.OpenElement(seq++, HtmlTag);
 
             // Splatted attributes.
-            foreach (var attribute in UserAttributes)
-            {
-                // Check if the attribute value is not null before adding it to the builder.
-                // This avoids adding null event handlers, such as `@onmouseenter=@(Open ? HandleEnter : null)`.
-                // This is useful because Blazor always adds the attribute value and creates an EventCallback in normal HTML elements.
-                if (attribute.Value is not null)
-                {
-                    builder.AddAttribute(seq++, attribute.Key, attribute.Value);
-                }
-            }
+            builder.AddMultipleAttributes(seq++, UserAttributes!);
 
             // Add class and style attributes.
             builder.AddAttribute(seq++, "class", Class);