Fix .length on undefined WASM error (#6048)

e2e/playwright/lib/console-error-whitelist.ts

@@ -258,14 +258,6 @@ export const isErrorWhitelisted = (exception: Error) => {
       foundInSpec: 'e2e/playwright/testing-settings.spec.ts',
     },
     // TODO: fix this error in the code
-    {
-      name: 'TypeError',
-      message: "Cannot read properties of undefined (reading 'length')",
-      stack: '',
-      project: 'Google Chrome',
-      foundInSpec: '', // many tests are impacted by this error
-    },
-    // TODO: fix this error in the code
     {
       name: 'ReferenceError',
       message: '_testUtils is not defined',

rust/kcl-lib/src/engine/conn_wasm.rs

@@ -121,6 +121,13 @@ impl EngineConnection {
             }
         })?;
 
+        if value.is_null() || value.is_undefined() {
+            return Err(KclError::Engine(KclErrorDetails {
+                message: "Received null or undefined response from engine".into(),
+                source_ranges: vec![source_range],
+            }));
+        }
+
         // Convert JsValue to a Uint8Array
         let data = js_sys::Uint8Array::from(value);