Error loading fxml javafx8 - exception

Question

I am trying to load the following fxml file. I simply get an exception thrown pointing to line 19. I can load this in scenebuilder just fine, via Show preview in window menu item. At first I thought one couldn't have rectangles be children of a stackpane but that is the very example Oracle uses for javadoc and StackPane. Is there a known best practice to debug failures to load and fxml file? It flags the first child under reticlefilter stackpane.
The two children in question are the light blue rectangle with the yellow reticle.
<?xml version="1.0" encoding="UTF-8"?>
<?import org.cornova.javafx.*?>
<?import javafx.scene.shape.*?>
<?import javafx.geometry.*?>
<?import javafx.scene.text.*?>
<?import javafx.scene.control.*?>
<?import java.lang.*?>
<?import javafx.scene.layout.*?>
<VBox fx:id="sliceRealEstate" alignment="TOP_CENTER" onMouseDragged="#onMouseDragged" onMousePressed="#onMousePressed" onScroll="#onScroll" onScrollStarted="#onScrollStarted" pickOnBounds="false" stylesheets="#org/cornova/javafx/xpssdr.css" xmlns="http://javafx.com/javafx/8.0.40" xmlns:fx="http://javafx.com/fxml/1" fx:controller="org.cornova.xpssdr.controllers.SliceController">
<children>
<HBox fx:id="flagPole" pickOnBounds="false">
<children>
<Pane fx:id="flagPane" pickOnBounds="false" stylesheets="#org/cornova/javafx/xpssdr.css">
<children>
<StackPane fx:id="reticleFilter" alignment="BOTTOM_LEFT" pickOnBounds="false">
<children>
***<Rectangle fx:id="filter" fill="#3670ad" height="40.0" opacity="0.38" width="30.0" StackPane.alignment="CENTER" />***
<Rectangle fx:id="reticle" fill="#e8e82f" height="40.0" width="2.0" StackPane.alignment="CENTER" />
</children>
</StackPane>
<HBox fx:id="flag">
<children>
<VBox alignment="TOP_CENTER" spacing="10.0" style="-fx-background-color: black;" HBox.hgrow="NEVER">
<children>
<Button fx:id="close" mnemonicParsing="false" onAction="#close" text="cls">
<font>
<Font size="10.0" />
</font>
</Button>
<Button fx:id="lock" layoutX="10.0" layoutY="10.0" mnemonicParsing="false" onAction="#lock" text="lck">
<font>
<Font size="10.0" />
</font>
</Button>
<Button fx:id="record" layoutX="10.0" layoutY="29.0" mnemonicParsing="false" onAction="#record" text="rec">
<font>
<Font size="10.0" />
</font>
</Button>
<Button fx:id="play" layoutX="10.0" layoutY="48.0" mnemonicParsing="false" onAction="#play" text="play">
<font>
<Font size="10.0" />
</font>
</Button>
</children>
</VBox>
<VBox alignment="CENTER" style="-fx-background-color: black;" HBox.hgrow="NEVER">
<children>
<HBox alignment="CENTER" VBox.vgrow="ALWAYS">
<children>
<VBox alignment="CENTER" HBox.hgrow="NEVER">
<children>
<HBox alignment="CENTER" VBox.vgrow="NEVER">
<children>
<VBox alignment="TOP_CENTER">
<children>
<ComboBox fx:id="rcvsrc" prefWidth="65.0" />
<ComboBox fx:id="txsrc" prefWidth="65.0" />
</children>
</VBox>
<Label fx:id="filterwidth" text="200" HBox.hgrow="NEVER" />
</children>
</HBox>
<HBox alignment="CENTER_LEFT" fillHeight="false" VBox.vgrow="NEVER">
<children>
<Label fx:id="nbon" text="nb" />
<Label fx:id="apfon" text="nr" />
<Label fx:id="nron" text="apf" />
<Label fx:id="qskon" text="qsk" />
</children>
</HBox>
</children>
</VBox>
<HBox HBox.hgrow="ALWAYS">
<children>
<Label fx:id="txindicator" alignment="CENTER" contentDisplay="CENTER" graphicTextGap="0.0" styleClass="largeindicators" text="TX" textAlignment="CENTER" textFill="RED">
<font>
<Font size="28.0" />
</font>
</Label>
<Label fx:id="sliceind" alignment="CENTER" contentDisplay="CENTER" styleClass="largeindicators" text="A" textAlignment="CENTER">
<font>
<Font size="27.0" />
</font>
</Label>
</children>
</HBox>
</children>
</HBox>
<VBox VBox.vgrow="NEVER">
<children>
<HBox alignment="CENTER" VBox.vgrow="NEVER">
<children>
<Label fx:id="signalLevel" alignment="CENTER" styleClass="mediumindicators" text="S9" HBox.hgrow="NEVER">
<opaqueInsets>
<Insets />
</opaqueInsets>
</Label>
<Label fx:id="signalplus" text="+20" />
<TextField fx:id="frequency" maxHeight="-Infinity" maxWidth="161.0" onAction="#onFrequencyChange" onMouseEntered="#scale" prefHeight="40.0" prefWidth="144.0" promptText="144.123456" styleClass="mediumindicators" />
</children>
</HBox>
</children>
</VBox>
<HBox VBox.vgrow="NEVER">
<children>
<ButtonBar buttonMinWidth="30.0" HBox.hgrow="NEVER">
<buttons>
<Button fx:id="spkr" mnemonicParsing="false" onAction="#displaySpkrSubmenu" text="spkr">
<font>
<Font size="10.0" />
</font>
</Button>
<Button fx:id="dsp" mnemonicParsing="false" onAction="#displayDSPSubmenu" text="dsp">
<font>
<Font size="10.0" />
</font>
</Button>
<Button fx:id="mode" layoutX="125.0" layoutY="17.0" mnemonicParsing="false" onAction="#displayModeSubmenu" text="rtty">
<font>
<Font size="10.0" />
</font>
</Button>
<Button layoutX="220.0" layoutY="17.0" mnemonicParsing="false" onAction="#displayXRITSubmenu" text="x/rit" fx:id="xrit">
<font>
<Font size="10.0" />
</font>
</Button>
<Button fx:id="dax" layoutX="315.0" layoutY="17.0" mnemonicParsing="false" onAction="#displayDAXSubmenu" text="dax">
<font>
<Font size="10.0" />
</font>
</Button>
</buttons>
<padding>
<Insets bottom="5.0" right="5.0" top="5.0" />
</padding>
</ButtonBar>
</children>
</HBox>
</children>
</VBox>
</children>
</HBox>
</children>
</Pane>
</children>
</HBox>
</children>
</VBox>
Additional information:
Exact text from exception
11:11:24.191 [nioEventLoopGroup-2-3] DEBUG org.cornova.xpssdr.views.SliceView - javafx.fxml.LoadException:
file:/home/walt/NetBeansProjects/XPSSDR/dist/run293699722/XPSSDR.jar!/SliceFlag.fxml:19
Code that does the load.
public VBox create() throws IOException {
VBox pane = null;
try {
pane = sliceLoader.load();
controller = sliceLoader.getController();
controller.setRadio(radio);
controller.setPan(pan);
controller = sliceLoader.getController();
controller.setPanadapter(pan);
controller.setParent(this);
controller.setSlice(slice);
wire();
Platform.runLater(() -> {
controller.wire();
});
} catch (Exception e) {
if (log.isDebugEnabled()) {
log.debug(e.getLocalizedMessage());
log.debug(e.getMessage());
log.debug(e.toString());
}
}
return pane;
}
The error was actually in the constructor to the controller. Late last night I, in an attempt to get the reticle displayed, when it was a ResizableCanvas I decided to change it to Rectangle such that it would simply take a fill(). As this was an existing app and existing controller that worked, such as it was, I didn't create it from SceneBuilder's View sample controller and had inadvertantly not changed the #FXML in the constructor of the controller. In FXMLLoader.java I was eventually able to trace it to the point it was trying to invoke something, the controller. Oddly though, it was pointing at the wrong controller, which I did verify was correct in the fxml. A puzzlement but not the current one. To you comment James about there is always a stack trace. No, many times there is not. It just strikes me perhaps I am missing some best practice on debugging these (JavaFX fxml loader issues which I've had several of these last couple of days.