A DESCRIPTION OF THE PROBLEM :
If you try to start an swt-application via webstart you get the error

This is my jnlp
<?xml version="1.0" encoding="utf-8"?>
<jnlp spec="1.0+" codebase="file:///Users/mkoch/Documents/VMs/eclipse_projects/Webstart">
    <information>
        
        <vendor>me</vendor>
    </information>
<security>
<all-permissions />
</security>
    <resources>
     <!-- add -XstartOnFirstThread to the java-vm-args (don't forget APPLICATION_TEMPLATE.JNLP -->
     <j2se version="1.8+" java-vm-args="" />
     <nativelib href="lib/swt.jar" />
<jar href="webstart.jar" main="true" />
<jar href="lib/swt.jar" />
    </resources>
    <application-desc name="Webstart-Test" main-class="com.c4u.webstart.SWTTest">
     </application-desc>
</jnlp>

this is the exception (this is ok because swt says what is wrong)

***WARNING: Display must be created on main thread due to Cocoa restrictions. Use vmarg -XstartOnFirstThread
Juli 02, 2018 1:36:40 NACHM. com.c4u.webstart.SWTTest main
SCHWERWIEGEND: error
org.eclipse.swt.SWTException: Invalid thread access
at org.eclipse.swt.SWT.error(SWT.java:4552)
at org.eclipse.swt.SWT.error(SWT.java:4467)
at org.eclipse.swt.SWT.error(SWT.java:4438)
at org.eclipse.swt.widgets.Display.error(Display.java:1090)
at org.eclipse.swt.widgets.Display.createDisplay(Display.java:842)
at org.eclipse.swt.widgets.Display.create(Display.java:825)
at org.eclipse.swt.graphics.Device.<init>(Device.java:129)
at org.eclipse.swt.widgets.Display.<init>(Display.java:724)
at org.eclipse.swt.widgets.Display.<init>(Display.java:715)
at com.c4u.webstart.SWTTest.main(SWTTest.java:30)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.base/java.lang.reflect.Method.invoke(Unknown Source)
at jdk.javaws@10.0.1/com.sun.javaws.Launcher.executeApplication(Unknown Source)
at jdk.javaws@10.0.1/com.sun.javaws.Launcher.executeMainClass(Unknown Source)
at jdk.javaws@10.0.1/com.sun.javaws.Launcher.doLaunchApp(Unknown Source)
at jdk.javaws@10.0.1/com.sun.javaws.Launcher.run(Unknown Source)
at java.base/java.lang.Thread.run(Unknown Source)

If I change
<j2se version="1.8+" java-vm-args="" />
to
<j2se version="1.8+" java-vm-args="-XstartOnFirstThread" />
I don't get an error but the application did not start.

To ensure that this is not a swt issue I've created a second application that creates a simple awt-frame.
Without -XstartOnFirstThread the awt-app starts as expected - with -XstartOnFirstThread I see the download progress and after that nothing happens (no app, no error).

So in my opinion this is an issue while launching the application with activated -XstartOnFirstThead.
/Users/mkoch/Library/Application Support/Oracle/Java/Deployment/log contains not really helpful informations.

I have a small project with my testfiles that I can send you if you want.