ADDITIONAL SYSTEM INFORMATION :
ArchLinux, AdoptOpenJDK 15.0.1 OpenJFX 15.0.1, using Kotlin, but the source code below is in Java so it may be easier for you to reproduce.

A DESCRIPTION OF THE PROBLEM :
When dropping a directory or a file from the GNOME File manager, the drop works properly, but then the following gets displayed in the log:
Exception in thread "JavaFX Application Thread" java.lang.NullPointerException: Cannot read field "dragboard" because "this.this$0.dndGesture" is null
at javafx.graphics/javafx.scene.Scene$DropTargetListener.drop(Scene.java:2965)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler.lambda$handleDragDrop$2(GlassSceneDnDEventHandler.java:108)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler.handleDragDrop(GlassSceneDnDEventHandler.java:104)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleDragDrop$11(GlassViewEventHandler.java:766)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:412)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.handleDragDrop(GlassViewEventHandler.java:765)
at javafx.graphics/com.sun.glass.ui.View.handleDragDrop(View.java:713)
at javafx.graphics/com.sun.glass.ui.View.notifyDragDrop(View.java:1042)
at javafx.graphics/com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
at javafx.graphics/com.sun.glass.ui.gtk.GtkApplication.lambda$runLoop$11(GtkApplication.java:277)
at java.base/java.lang.Thread.run(Thread.java:832)


The event is properly consumed and isDropCompleted is set to true.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Drag a file or a directory from Gnome File manager onto a label that has onDragOver (selects only COPY/LINK orMOVE) , onDragDropped (that if it has files, set isDropCompleted to true and then consume the event, and a onDragExited that consume the event.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
No message displayed in the log
ACTUAL -
Message displayed in the log:

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException: Cannot read field "dragboard" because "this.this$0.dndGesture" is null
at javafx.graphics/javafx.scene.Scene$DropTargetListener.drop(Scene.java:2965)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler.lambda$handleDragDrop$2(GlassSceneDnDEventHandler.java:108)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassSceneDnDEventHandler.handleDragDrop(GlassSceneDnDEventHandler.java:104)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleDragDrop$11(GlassViewEventHandler.java:766)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:412)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.handleDragDrop(GlassViewEventHandler.java:765)
at javafx.graphics/com.sun.glass.ui.View.handleDragDrop(View.java:713)
at javafx.graphics/com.sun.glass.ui.View.notifyDragDrop(View.java:1042)
at javafx.graphics/com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
at javafx.graphics/com.sun.glass.ui.gtk.GtkApplication.lambda$runLoop$11(GtkApplication.java:277)
at java.base/java.lang.Thread.run(Thread.java:832)


---------- BEGIN SOURCE ----------
package sample;

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.input.*;
import javafx.scene.layout.VBox;
import javafx.scene.paint.Color;
import javafx.scene.text.Font;
import javafx.scene.text.Text;
import javafx.stage.Stage;

public class Main extends Application {

    @Override public void start(Stage stage) {
        VBox root = new VBox();
        Scene scene = new Scene(root, 400, 100);

        final Text target = new Text("DROP HERE");
        target.setFont(Font.font(64.0));

        target.setOnDragOver(event -> {
            System.out.println("onDragOver");
            if (event.getGestureSource() != target &&
                    event.getDragboard().hasFiles()) {
                event.acceptTransferModes(TransferMode.COPY_OR_MOVE);
            }
            event.consume();
        });

        target.setOnDragEntered(event -> {
            System.out.println("onDragEntered");
            if (event.getGestureSource() != target &&
                    event.getDragboard().hasFiles()) {
                target.setFill(Color.GREEN);
            }
            event.consume();
        });

        target.setOnDragExited(event -> {
            target.setFill(Color.BLACK);
            event.consume();
        });

        target.setOnDragDropped(event -> {
            System.out.println("onDragDropped");
            Dragboard db = event.getDragboard();
            boolean success = false;
            if (db.hasFiles()) {
                target.setText(String.valueOf(db.getFiles().size()));
                success = true;
            }
            event.setDropCompleted(success);
            event.consume();
        });

        root.getChildren().add(target);
        stage.setScene(scene);
        stage.show();
    }

    public static void main(String[] args) {
        Application.launch(args);
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
None found

FREQUENCY : always