Javafx error exception in Application start method no controller specified

The error says it, the FXML is missing the fx:controller declaration. Add the controller declaration to BorderPane declaration as shown :

...
<BorderPane maxHeight="1.7976931348623157E308" 
      maxWidth="1.7976931348623157E308" prefHeight="400.0" 
       prefWidth="600.0" xmlns="http://javafx.com/javafx/8"
         xmlns:fx="http://javafx.com/fxml/1" 
                          fx:controller="Weert.Controller">
...

To know how to set controller class via SceneBuilder, you can have a look at this question.