add Callback error test

tests/DemosTest.php

@@ -474,6 +474,32 @@ public function testDemoAssertJsonResponsePost(string $uri, array $postData): vo
         $this->assertSame(200, $response->getStatusCode(), ' Status error on ' . $uri);
         $this->assertMatchesRegularExpression($this->regexJson, $response->getBody()->getContents(), ' RegExp error on ' . $uri);
     }
+
+    public function testCallbackError(): void
+    {
+        if (static::class === self::class) {
+            $this->expectException(\Atk4\Ui\Exception::class);
+            $this->expectExceptionMessage('Callback requested, but never reached. You may be missing some arguments in request URL.');
+        }
+
+        $uri = 'obsolete/notify2.php?' . Callback::URL_QUERY_TRIGGER_PREFIX . 'test_notify=ajax&' . Callback::URL_QUERY_TARGET . '=callback_trigger_error';
+        $data = [
+            'text' => 'This text will appear in notification',
+            'icon' => 'warning sign',
+            'color' => 'green',
+            'transition' => 'jiggle',
+            'width' => '25%',
+            'position' => 'topRight',
+            'attach' => 'Body',
+        ];
+
+        try {
+            $this->getResponseFromRequest($uri, ['form_params' => $data]);
+        } catch (\GuzzleHttp\Exception\ServerException $e) {
+            $this->assertSame(500, $e->getResponse()->getStatusCode());
+            $this->assertStringContainsString('Callback requested, but never reached. You may be missing some arguments in request URL.', (string) $e->getResponse()->getBody());
+        }
+    }
 }
 
 class DemosTestExitError extends \Error