Make sure we only continue when Drupal core is already available

src/Handler.php

@@ -85,6 +85,10 @@ class Handler {
     $installationManager = $this->composer->getInstallationManager();
 
     $drupalCorePackage = $this->getDrupalCorePackage();
+    if (!$drupalCorePackage) {
+      // We are called too early, Drupal core is not available yet.
+      return;
+    }
     $corePath = $installationManager->getInstallPath($drupalCorePackage);
 
     // Directory where the root project is being created