Issue #3459288: Unable to add and install a module via the drupal.org (json:api) source type

src/Controller/InstallerController.php

@@ -337,13 +337,14 @@ class InstallerController extends ControllerBase {
    *   Status message.
    */
   public function begin(Project $project): JsonResponse {
-    // @todo Expand to support other plugins in https://drupal.org/i/3312354.
-    $source = $this->enabledSourceHandler->getCurrentSources()['drupalorg_mockapi'] ?? NULL;
-    if ($source === NULL) {
-      return new JsonResponse(['message' => "Cannot download $project->id from any available source"], 500);
-    }
-    if (!$source->isProjectSafe($project)) {
-      return new JsonResponse(['message' => "$project->machineName is not safe to add because its security coverage has been revoked"], 500);
+    if ($project->source === 'drupalorg_mockapi') {
+      $source = $this->enabledSourceHandler->getCurrentSources()[$project->source] ?? NULL;
+      if ($source === NULL) {
+        return new JsonResponse(['message' => "Cannot download $project->id from any available source"], 500);
+      }
+      if (!$source->isProjectSafe($project)) {
+        return new JsonResponse(['message' => "$project->machineName is not safe to add because its security coverage has been revoked"], 500);
+      }
     }
 
     $stage_available = $this->installer->isAvailable();