ADDITIONAL SYSTEM INFORMATION :
Tested on HighSierra (10.13.6) and (Catalina 10.15.3) and the application crashes on both.
Java 14
JavaFx 13, JavaFx 14

A DESCRIPTION OF THE PROBLEM :
When adding an event handler that is closing the dialog via CMD+ENTER or CTRL+ENTER key combination, this crashes the entire application on MacOs.

This does NOT happen on Windows or Linux.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Click launch dialog button (the dialog should open)
2. Press ALT+ENTER (the dialog should be closed)
3. Click launch dialog button again (the dialog should open)
4. Press CMD+ENTER
5. The application will crash

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Application should not crash.
ACTUAL -
2020-04-09 20:18:49.116 java[788:13938] CGLChoosePixelFormat: No matching pixel format exists for the requested attributes, trying again with limited capabilities
ALT + ENTER
Apply processed
2020-04-09 20:18:52.754 java[788:13938] CGLChoosePixelFormat: No matching pixel format exists for the requested attributes, trying again with limited capabilities
META + ENTER
Apply processed
2020-04-09 20:18:53.838 java[788:13938] *** Terminating app due to uncaught exception 'NSRangeException', reason: '*** -[__NSArrayM objectAtIndex:]: index 1 beyond bounds [0 .. 0]'
*** First throw call stack:
(
0 CoreFoundation 0x00007fff563cd66b __exceptionPreprocess + 171
1 libobjc.A.dylib 0x00007fff7d6da942 objc_exception_throw + 48
2 CoreFoundation 0x00007fff5640d77c _CFThrowFormattedException + 194
3 CoreFoundation 0x00007fff562f008f -[__NSArrayM objectAtIndex:] + 175
4 AppKit 0x00007fff53b041a8 -[NSView performKeyEquivalent:] + 172
5 AppKit 0x00007fff541ab0f7 -[NSWindow _commonPerformKeyEquivalent:conditionally:] + 79
6 libglass.dylib 0x0000000126a1ae30 -[GlassWindow_Normal performKeyEquivalent:] + 64
7 AppKit 0x00007fff540704fb routeKeyEquivalent + 454
8 AppKit 0x00007fff5406dce0 -[NSApplication(NSEvent) sendEvent:] + 1096
9 libglass.dylib 0x0000000126a0a435 +[GlassApplication enterNestedEventLoopWithEnv:] + 213
10 libglass.dylib 0x0000000126a0aeaa Java_com_sun_glass_ui_mac_MacApplication__1enterNestedEventLoopImpl + 74
11 ??? 0x00000001119e16b0 0x0 + 4590540464
)
libc++abi.dylib: terminating with uncaught exception of type NSException
[ERROR] Command execution failed.
org.apache.commons.exec.ExecuteException: Process exited with an error: 134 (Exit value: 134)

---------- BEGIN SOURCE ----------
import javafx.application.Application;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.input.KeyCode;
import javafx.scene.input.KeyEvent;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.StackPane;
import javafx.scene.layout.VBox;
import javafx.stage.Modality;
import javafx.stage.Stage;
import javafx.stage.StageStyle;

public class CrashMe extends Application {

    @Override
    public void start(Stage stage) {
        ////////////////////////////////////////
        // create main screen
        ////////////////////////////////////////
        Button launchButton = new Button("Launch dialog");
        VBox vbox = new VBox(launchButton);
        Scene scene = new Scene(new StackPane(vbox), 640, 480);
        stage.setScene(scene);

        ////////////////////////////////////////
        // dialog which crashes the application
        ////////////////////////////////////////
        Button clickMe = new Button("Close dialog");
        BorderPane pane = new BorderPane(clickMe);
        Dialog dialog = new Dialog(stage, pane);
        Runnable dialogApply = () -> {
            System.out.println("Apply processed");
            dialog.close();
        };
        dialog.setApply(dialogApply);
        clickMe.setOnAction(e -> dialogApply.run());

        ////////////////////////////////////////
        // wire button on main screen to launch the dialog
        ////////////////////////////////////////
        launchButton.setOnAction(e -> {
            dialog.showAndWait();
        });
        stage.show();
    }

    public static void main(String[] args) {
        launch();
    }

    public static class Dialog extends Stage {
        private Runnable apply;

        public Dialog(Stage owner, Parent layout) {
            super(StageStyle.DECORATED);
            Scene layoutScene = new Scene(layout, 500, 500);
            this.setScene(layoutScene);

            layoutScene.addEventHandler(KeyEvent.KEY_PRESSED, event -> {
                if (event.getCode() == KeyCode.ENTER && event.isShortcutDown()) {
                    // this crashes the app on MacOs
                    System.out.println("META + ENTER");
                    if (apply != null) {
                        apply.run();
                    }
                    event.consume();
                    return;
                }

                if (event.getCode() == KeyCode.ENTER && event.isAltDown()) {
                    // this does not crash the app on MacOs
                    System.out.println("ALT + ENTER");
                    if (apply != null) {
                        apply.run();
                    }
                    event.consume();
                    return;
                }
            });

            this.hide();
            this.initModality(Modality.APPLICATION_MODAL);
            this.initOwner(owner);
            this.setResizable(true);
        }

        public void setApply(Runnable apply) {
            this.apply = apply;
        }

        @Override
        public void close() {
            super.close();
        }
    }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
ALT+ENTER does not crash the application
Clicking directly on button does not crash the application

FREQUENCY : always