add null check

Oqtane.Client/UI/ThemeBuilder.razor

@@ -65,6 +65,8 @@
     }
 
     private string AddHeadContent(string headcontent, string content)
+    {
+        if (!string.IsNullOrEmpty(content))
         {
             // format head content, remove scripts, and filter duplicate elements
             var elements = (">" + content.Replace("\n", "") + "<").Split("><");
@@ -78,6 +80,7 @@
                     }
                 }
             }
+        }
         return headcontent;
     }