Strange behavoir on dialog

I have some issue with an dialog in my application. In the dialog, I have a few checkboxes. When the user changes the state of a checkbox, stuff happening in the backend, and the front end should present a loading-spinner. To achive this, I have a .fxml with the loading-image, which is added to the dialog with the checkboxes.

The whole loading process and the background process works out just fine. Even the image of the loading spinner are showing as it should. But, before the loading-image is visible, dialog disappears and then return again with the loading-spinner. It looks like a styled minimizing of the dialog...

Any ideas why?

Thanks in advance

Loading.fxml

<GridPane  xmlns:fx="http://javafx.com/fxml"
       styleClass="foo-box"
       stylesheets="/styles/style.css">

<BorderPane>
    <center>
        <ImageView  fitHeight="45" fitWidth="45" pickOnBounds="true" preserveRatio="true">
            <image>
                <Image preserveRatio="true" smooth="true" url="/loading.gif" />
            </image>
        </ImageView>
    </center>
</BorderPane>

To show the loading-image, i use show() method...