test that error is thrown if plan doesnt exist for creating scheduled node

app/ScheduledNode/UseCases/CreateScheduledNode.php

@@ -6,6 +6,7 @@ use App\Plan\PlanRepository;
 use App\ScheduledNode\ScheduledNode;
 use App\ScheduledNode\CreateScheduledNodeDto;
 use App\ScheduledNode\ScheduledNodeRepository;
+use DomainException;
 
 class CreateScheduledNode
 {
@@ -17,7 +18,12 @@ class CreateScheduledNode
     public function execute(
         CreateScheduledNodeRequest $request
     ): ScheduledNode {
-        $plan = $this->planRepo->find($request->planId);
+        $id = $request->planId;
+        $plan = $this->planRepo->find($id);
+        if ($plan === null) {
+            throw new DomainException("Plan with id: $id doesnt exist");
+        }
+
         return $this->scheduledNodeRepo->create(
             new CreateScheduledNodeDto(
                 date: $request->date,

tests/Unit/ScheduledNode/UseCases/CreateScheduledNodeTest.php

@@ -11,6 +11,7 @@ use App\ScheduledNode\UseCases\CreateScheduledNodeRequest;
 use App\User\User;
 use App\ValueObjects\EmailAddress;
 use DateTimeImmutable;
+use DomainException;
 use Tests\Fakes\FakePlanRepository;
 use Tests\Fakes\FakeScheduledNodeRepository;
 use PHPUnit\Framework\TestCase;
@@ -62,4 +63,16 @@ class CreateScheduledNodeTest extends TestCase
         );
         $this->assertInstanceOf(Plan::class, $scheduledNode->getPlan());
     }
+
+    public function test_nonexistant_plan_throws(): void
+    {
+        $this->expectException(DomainException::class);
+        $this->expectExceptionMessage('Plan with id: 1 doesnt exist');
+        $this->useCase->execute(
+            new CreateScheduledNodeRequest(
+                date: new DateTimeImmutable('now'),
+                planId: 1,
+            )
+        );
+    }
 }