Improve error message when attempting to fork an unforkable component.

Fixes #5142 and rainlab/forum-plugin#141

modules/cms/controllers/Index.php

@@ -403,6 +403,11 @@ class Index extends Controller
         $manager = ComponentManager::instance();
         $componentObj = $manager->makeComponent($componentName);
         $partial = ComponentPartial::load($componentObj, 'default');
+
+        if (!$partial) {
+            throw new ApplicationException(Lang::get('cms::lang.component.no_default_partial'));
+        }
+
         $content = $partial->getContent();
         $content = str_replace('__SELF__', $alias, $content);
 

modules/cms/lang/en/lang.php

@@ -249,6 +249,7 @@ return [
         'invalid_request' => 'The template cannot be saved because of invalid component data.',
         'no_records' => 'No components found',
         'not_found' => "The component ':name' is not found.",
+        'no_default_partial' => "This component does not have a 'default' partial",
         'method_not_found' => "The component ':name' does not contain a method ':method'.",
         'soft_component' => 'Soft Component',
         'soft_component_description' => 'This component is missing but optional.',