Make sure each closure returns a valid response

src/Mono.php

@@ -100,8 +100,11 @@ public function run(): void
             default => $this->createResponse('500 Internal Server Error', 500)
         };
 
-        if ($response instanceof ResponseInterface) {
-            (new SapiEmitter())->emit($response);
+        if (!$response instanceof ResponseInterface) {
+            throw new \RuntimeException('Invalid response received from route ' . $request->getUri()->getPath() .
+                '. Please return a valid PSR-7 response from your handler.');
         }
+
+        (new SapiEmitter())->emit($response);
     }
 }

tests/MonoTest.php

@@ -83,8 +83,26 @@ public function testDependencyInjection(): void
             $demoClass = $mono->get(NodeTraverser::class);
 
             $this->assertInstanceOf(NodeTraverser::class, $demoClass);
+
+            return $mono->createResponse('OK');
         });
 
         $mono->run();
     }
+
+    public function testInvalidResponseThrowError(): void
+    {
+        $_SERVER['REQUEST_METHOD'] = 'GET';
+        $_SERVER['REQUEST_URI'] = '/';
+
+        $mono = new Mono(__DIR__);
+
+        $mono->addRoute('GET', '/', function () {
+            return 'OK';
+        });
+
+        $this->expectException(\RuntimeException::class);
+
+        $mono->run();
+    }
 }