diff -u b/src/Form/SensorForm.php b/src/Form/SensorForm.php
--- b/src/Form/SensorForm.php
+++ b/src/Form/SensorForm.php
@@ -109,12 +109,6 @@
     );
 
     if (isset($sensor_config->plugin_id) && $plugin = $sensor_config->getPlugin()) {
-
-      // Set default configuration of the sensor.
-      if ($default_config = $plugin->getDefaultConfiguration()) {
-        $sensor_config->set('settings', $default_config);
-      }
-
       $form['plugin_container']['settings'] = array(
         '#type' => 'details',
         '#open' => TRUE,
@@ -216,6 +210,11 @@
    */
   public function submitSelectPlugin(array $form, FormStateInterface $form_state) {
     $this->entity = $this->buildEntity($form, $form_state);
+
+    // Set default configuration of the sensor.
+    if ($default_config = $this->entity->getPlugin()->getDefaultConfiguration()) {
+      $this->entity->set('settings', $default_config);
+    }
     $form_state->setRebuild();
   }
 
diff -u b/src/Plugin/monitoring/SensorPlugin/WatchdogAggregatorSensorPlugin.php b/src/Plugin/monitoring/SensorPlugin/WatchdogAggregatorSensorPlugin.php
--- b/src/Plugin/monitoring/SensorPlugin/WatchdogAggregatorSensorPlugin.php
+++ b/src/Plugin/monitoring/SensorPlugin/WatchdogAggregatorSensorPlugin.php
@@ -56,9 +56,7 @@
   public function getDefaultConfiguration() {
     $default_config = array(
       'table' => 'watchdog',
-      'aggregation' => array(
-        'time_interval_field' => 'timestamp',
-      ),
+      'time_interval_field' => 'timestamp',
     );
     return $default_config;
   }