Bug 1767829 - Part 4: Throw an error if CreateDynamicImport returns null r=yulia

This allows derived loaders not to support dynamic import if they don't want to.

Module loading for in mozJSComponentLoader will not initially support this.

Differential Revision: https://phabricator.services.mozilla.com/D145491

js/loader/ModuleLoaderBase.cpp

@@ -222,6 +222,11 @@ bool ModuleLoaderBase::HostImportModuleDynamically(
   RefPtr<ModuleLoadRequest> request = loader->CreateDynamicImport(
       aCx, uri, script, aReferencingPrivate, specifierString, aPromise);
 
+  if (!request) {
+    JS_ReportErrorASCII(aCx, "Dynamic import not supported in this context");
+    return false;
+  }
+
   loader->StartDynamicImport(request);
   return true;
 }