A DESCRIPTION OF THE PROBLEM :
The bug I report seems to be fixed in JDK-8211304, but only in Java 12. Please fix it also in Java 11.
This is a major bug for all JavaFx applications running on MacOs.
This is a Java crash on Mac OS Mojave, the stack trace is below.
I have also attached a small application you can run. Just open the dialog and then switch few times to another application ( alt-tab). The crash will reproduce.

018-12-09 08:47:08.653 java[13845:734283] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Invalid parameter not satisfying: _type > 0 && _type <= kCGSLastEventType'
*** First throw call stack:
(
	0   CoreFoundation                      0x00007fff520b543d __exceptionPreprocess + 256
	1   libobjc.A.dylib                     0x00007fff7dfc2720 objc_exception_throw + 48
	2   CoreFoundation                      0x00007fff520d008e +[NSException raise:format:arguments:] + 98
	3   Foundation                          0x00007fff5446c55d -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 194
	4   AppKit                              0x00007fff4f55bacd -[NSEvent _initWithCGEvent:eventRef:] + 3272
	5   AppKit                              0x00007fff4f8af188 +[NSEvent eventWithCGEvent:] + 120
	6   libglass.dylib                      0x000000012467bf3b listenTouchEvents + 59
	7   SkyLight                            0x00007fff78080782 _ZL19processEventTapDataPvjjjPhj + 148
	8   SkyLight                            0x00007fff77f7fbb2 _XPostEventTapData + 278
	9   SkyLight                            0x00007fff78080690 _ZL22eventTapMessageHandlerP12__CFMachPortPvlS1_ + 132
	10  CoreFoundation                      0x00007fff52013a5b __CFMachPortPerform + 246
	11  CoreFoundation                      0x00007fff52013959 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 41
	12  CoreFoundation                      0x00007fff520138b7 __CFRunLoopDoSource1 + 527
	13  CoreFoundation                      0x00007fff51ffb945 __CFRunLoopRun + 2574
	14  CoreFoundation                      0x00007fff51fface4 CFRunLoopRunSpecific + 463
	15  Foundation                          0x00007fff5435b5da -[NSRunLoop(NSRunLoop) runMode:beforeDate:] + 280
	16  libglass.dylib                      0x00000001246709f5 +[GlassApplication enterNestedEventLoopWithEnv:] + 165
	17  libglass.dylib                      0x000000012467148a Java_com_sun_glass_ui_mac_MacApplication__1enterNestedEventLoopImpl + 74
	18  ???                                 0x000000010793a950 0x0 + 4422084944
	19  ???                                 0x0000000107934a00 0x0 + 4422060544
	20  ???                                 0x0000000107934a00 0x0 + 4422060544
	21  ???                                 0x0000000107934a00 0x0 + 4422060544
)
libc++abi.dylib: terminating with uncaught exception of type NSException




---------- BEGIN SOURCE ----------

import javafx.application.Application;
import javafx.scene.Node;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.ButtonType;
import javafx.scene.control.Dialog;
import javafx.scene.layout.StackPane;
import javafx.scene.layout.VBox;
import javafx.stage.Modality;
import javafx.stage.Stage;

public class SampleAppWithButton extends Application {


    private final StackPane root = new StackPane();
    private final Scene scene = new Scene(root, 300, 250);

    @Override
    public void start(Stage stage) {
        stage.setTitle("Sample Canvas");

        final Button btn = new Button("Show Dialog");
        btn.setOnAction(e -> showDialog());

        VBox pane = new VBox();
        pane.getChildren().addAll(btn);

        root.getChildren().add(pane);
        stage.setScene(scene);
        stage.sizeToScene();
        stage.show();

    }

    private void showDialog() {
        final Dialog dialog = new Dialog();
        dialog.initModality(Modality.APPLICATION_MODAL);
        dialog.initOwner(scene.getWindow());
        dialog.setResizable(true);
        dialog.setTitle("UID Setup");
        dialog.getDialogPane().getButtonTypes().addAll(ButtonType.OK, ButtonType.CANCEL);
        dialog.showAndWait();

        Node okButton = dialog.getDialogPane().lookupButton(ButtonType.OK);

    }

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


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

FREQUENCY : often