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.

As the error implies, problem is in "fxml", in which it is referring to class which do not exist. To solve this search an attribute called "fx:controller" and assign it qualified class name . For an instance "fx:controller="packagename.className".