fix: catalog:fetch to throw error for null entity

fixes #16447

Signed-off-by: Heikki Hellgren <heikki.hellgren@op.fi>

.changeset/eight-radios-bake.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-scaffolder-backend': patch
+---
+
+throw error from catalog:fetch scaffolder action when entity is null and optional is false

plugins/scaffolder-backend/src/scaffolder/actions/builtin/catalog/fetch.test.ts

@@ -90,7 +90,7 @@ describe('catalog:fetch', () => {
     expect(mockContext.output).toHaveBeenCalledWith('entity', null);
   });
 
-  it('should throw error if entity not in catalog and optional is false', async () => {
+  it('should throw error if entity fetch fails from catalog and optional is false', async () => {
     getEntityByRef.mockImplementationOnce(() => {
       throw new Error('Not found');
     });
@@ -109,4 +109,22 @@ describe('catalog:fetch', () => {
     });
     expect(mockContext.output).not.toHaveBeenCalled();
   });
+
+  it('should throw error if entity not in catalog and optional is false', async () => {
+    getEntityByRef.mockReturnValueOnce(null);
+
+    await expect(
+      action.handler({
+        ...mockContext,
+        input: {
+          entityRef: 'component:default/test',
+        },
+      }),
+    ).rejects.toThrow('Entity component:default/test not found');
+
+    expect(getEntityByRef).toHaveBeenCalledWith('component:default/test', {
+      token: 'secret',
+    });
+    expect(mockContext.output).not.toHaveBeenCalled();
+  });
 });

plugins/scaffolder-backend/src/scaffolder/actions/builtin/catalog/fetch.ts

@@ -93,6 +93,11 @@ export function createFetchCatalogEntityAction(options: {
           throw e;
         }
       }
+
+      if (!entity && !optional) {
+        throw new Error(`Entity ${entityRef} not found`);
+      }
+
       ctx.output('entity', entity ?? null);
     },
   });