add error log when root element isn't found (#1724)

packages/web/Cargo.toml

@@ -42,6 +42,7 @@ features = [
     "HtmlFormElement",
     "Text",
     "Window",
+    "console",
 ]
 
 [features]

packages/web/src/dom.rs

@@ -45,7 +45,16 @@ impl WebsysDom {
         let document = load_document();
         let root = match document.get_element_by_id(&cfg.rootname) {
             Some(root) => root,
-            None => document.create_element("body").ok().unwrap(),
+            None => {
+                web_sys::console::error_1(
+                    &format!(
+                        "element '#{}' not found. mounting to the body.",
+                        cfg.rootname
+                    )
+                    .into(),
+                );
+                document.create_element("body").ok().unwrap()
+            }
         };
         let interpreter = Channel::default();