title: JavaFX Node.lookup() returning null only for some elements in Parent loaded with FXMLLoader
body: <p>I've loaded a Parent from FXML, added it to a Pane within a scene/stage and shown it, then immediately afterwards I lookup components. lookup() returns null for some but non-null for others. Under what circumstances would it do that?</p>
  
  <p>Here's the loading and lookup code:</p>
  
  <pre><code>    rootUi = FXMLLoader.load(getClass().getResource("PracticeScreen.fxml"));
      // added to Parent within stage and setVisible(true)
      analysisGroup = (Pane)rootUi.lookup("#analysisGroup");       // null
      stickiesPane = (Pane)rootUi.lookup("#stickiesPane");         // null
      scoreScroll = (ScrollPane)rootUi.lookup("#scoreScrollPane"); // GOOD
      tintLayer = rootUi.lookup("#tintLayer");                     // GOOD
      scoreImageView = (ImageView)rootUi.lookup("#scoreImage");    // null
      StackPane scoreRenderStack = (StackPane)rootUi.lookup("#scoreRenderStack"); // null
      StackPane scoreScrollStack = (StackPane)rootUi.lookup("#scoreScrollStack"); // null
  </code></pre>
  
  <p>scoreScrollPane comes back OK, but then all of its children come back null.</p>
  
  <p>FXML loaded:</p>
  
  
  
  <pre><code>  &lt;?import javafx.geometry.Insets?&gt;
    &lt;?import javafx.scene.control.Label?&gt;
    &lt;?import javafx.scene.control.ScrollPane?&gt;
    &lt;?import javafx.scene.control.ToggleButton?&gt;
    &lt;?import javafx.scene.effect.DropShadow?&gt;
    &lt;?import javafx.scene.effect.Glow?&gt;
    &lt;?import javafx.scene.image.ImageView?&gt;
    &lt;?import javafx.scene.layout.*?&gt;
    &lt;?import javafx.scene.shape.Rectangle?&gt;
    &lt;?import javafx.scene.text.Font?&gt;
    &lt;BorderPane style="-fx-background-color: white;" xmlns="http://javafx.com/javafx/8.0.65" xmlns:fx="http://javafx.com/fxml/1"&gt;
       &lt;center&gt;
          &lt;StackPane id="scoreScreenStack" style="-fx-background-color: blue;" BorderPane.alignment="CENTER"&gt;
             &lt;children&gt;
                &lt;ScrollPane id="scoreScrollPane" fitToHeight="true" hbarPolicy="ALWAYS" vbarPolicy="NEVER"&gt;
                   &lt;content&gt;
                      &lt;StackPane id="scoreScrollStack"&gt;
                         &lt;children&gt;
                            &lt;StackPane id="scoreRenderStack" alignment="CENTER_LEFT" StackPane.alignment="CENTER_LEFT"&gt;
                               &lt;children&gt;
                                  &lt;ImageView id="scoreImage" pickOnBounds="true" preserveRatio="true" StackPane.alignment="CENTER_LEFT" /&gt;
                                  &lt;Pane id="analysisGroup" /&gt;
                                  &lt;Pane id="stickiesPane" minHeight="200.0" minWidth="200.0" /&gt;
                                  &lt;Rectangle id="playbackCursor" arcHeight="5.0" arcWidth="5.0" fill="DODGERBLUE" height="150.0" stroke="BLACK" strokeType="INSIDE" strokeWidth="0.0" visible="false" width="2.0"&gt;
                                     &lt;effect&gt;
                                        &lt;DropShadow color="DODGERBLUE" /&gt;
                                     &lt;/effect&gt;
                                  &lt;/Rectangle&gt;
                               &lt;/children&gt;
                            &lt;/StackPane&gt;
                         &lt;/children&gt;
                      &lt;/StackPane&gt;
                   &lt;/content&gt;
                &lt;/ScrollPane&gt;
                &lt;BorderPane pickOnBounds="false"&gt;
                   &lt;top&gt;
                      &lt;StackPane BorderPane.alignment="CENTER"&gt;
                         &lt;children&gt;
                            &lt;BorderPane&gt;
                               &lt;left&gt;
                                  &lt;Label id="recordingIndicator" text="Listening..." textFill="#00a7ff" BorderPane.alignment="CENTER"&gt;
                                     &lt;font&gt;
                                        &lt;Font size="18.0" /&gt;
                                     &lt;/font&gt;
                                     &lt;effect&gt;
                                        &lt;Glow level="0.86" /&gt;
                                     &lt;/effect&gt;
                                     &lt;BorderPane.margin&gt;
                                        &lt;Insets /&gt;
                                     &lt;/BorderPane.margin&gt;
                                     &lt;padding&gt;
                                        &lt;Insets bottom="8.0" left="8.0" right="8.0" top="8.0" /&gt;
                                     &lt;/padding&gt;
                                  &lt;/Label&gt;
                               &lt;/left&gt;
                               &lt;right&gt;
                                  &lt;HBox BorderPane.alignment="CENTER"&gt;
                                     &lt;children&gt;
                                        &lt;ToggleButton id="finalPerformance" mnemonicParsing="false" text="Final Performance" /&gt;
                                     &lt;/children&gt;
                                     &lt;BorderPane.margin&gt;
                                        &lt;Insets bottom="8.0" left="8.0" right="8.0" top="8.0" /&gt;
                                     &lt;/BorderPane.margin&gt;
                                  &lt;/HBox&gt;
                               &lt;/right&gt;
                            &lt;/BorderPane&gt;
                         &lt;/children&gt;
                      &lt;/StackPane&gt;
                   &lt;/top&gt;
                &lt;/BorderPane&gt;
                &lt;Pane id="tintLayer" opacity="0.0" pickOnBounds="false" style="-fx-background-color: #00a7ff;" visible="false" /&gt;
             &lt;/children&gt;
          &lt;/StackPane&gt;
       &lt;/center&gt;
    &lt;/BorderPane&gt;
  </code></pre>