Swing GUI Builder crashes - ClassNotFoundException: java.applet.Applet on JDK 26

[nz-cloud]

Apache NetBeans version

Apache NetBeans 29

What happened

I am using NetBeans on CachyOS (Arch-based Linux).

When trying to use the Swing GUI Builder (Design tab), I get an exception:

java.lang.ClassNotFoundException: java.applet.Applet

This happens when adding components in Design view.

Java version:
openjdk 26.0.2

Expected behavior:
GUI Builder should allow adding components normally.

Actual behavior:
Unexpected Exception dialog appears and components cannot be added.

messages.log attached.

Language / Project Type / NetBeans Component

Java with Ant

How to reproduce

Steps to reproduce:

1. Create JFrame Form
2. Open Design tab
3. Try to add a Swing component
4. Exception occurs

messages.log

Did this work correctly in an earlier version?

No / Don't know

Operating System

CachyOS (Arch-based Linux)

JDK

openjdk 26.0.2

Apache NetBeans packaging

Other

Anything else

No response

Are you willing to submit a pull request?

No

[mbien]

from log:

>Log Session: Wednesday, April 29, 2026, 11:33:06 PM Brasilia Standard Time
>System Info: 
  Product Version         = Apache NetBeans IDE 26

this issue incorrectly states NB 29

please try the current release. NB 26 does not support running on JDK 26. The supported version range is always mentioned on the download page of the release.

[nz-cloud]

HI,
I switched from JDK 26 to JDK 21 and then I tested my previous issue in NB 26 runnnig on JDK 21 and it worked.
Thank you for your response.