Error when installing SOAP UI on Ubuntu

I get the following error:

$ ./SoapUI-x64-5.6.0.sh      
Starting Installer ...
java.lang.NoClassDefFoundError: Could not initialize class java.awt.Container
       at com.install4j.runtime.installer.frontend.headless.AbstractHeadlessScreenExecutor.init(AbstractHeadlessScreenExecutor.java:68)
       at com.install4j.runtime.installer.frontend.headless.ConsoleScreenExecutor.<init>(ConsoleScreenExecutor.java:24)
       at com.install4j.runtime.installer.frontend.headless.InstallerConsoleScreenExecutor.<init>(InstallerConsoleScreenExecutor.java:6)
       at com.install4j.runtime.installer.Installer.getScreenExecutor(Installer.java:88)
       at com.install4j.runtime.installer.Installer.runInProcess(Installer.java:57)
       at com.install4j.runtime.installer.Installer.main(Installer.java:45)
       at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
       at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
       at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
       at java.base/java.lang.reflect.Method.invoke(Method.java:566)
       at com.exe4j.runtime.LauncherEngine.launch(LauncherEngine.java:85)
       at com.install4j.runtime.launcher.UnixLauncher.main(UnixLauncher.java:62)
An error occurred:
java.lang.NoClassDefFoundError: Could not initialize class java.awt.Container
Error log: /tmp/install4jError3834593080281114764.log
java.lang.NoClassDefFoundError: Could not initialize class java.awt.Container
       at com.install4j.runtime.installer.frontend.headless.AbstractHeadlessScreenExecutor.init(AbstractHeadlessScreenExecutor.java:68)
       at com.install4j.runtime.installer.frontend.headless.ConsoleScreenExecutor.<init>(ConsoleScreenExecutor.java:24)
       at com.install4j.runtime.installer.frontend.headless.InstallerConsoleScreenExecutor.<init>(InstallerConsoleScreenExecutor.java:6)
       at com.install4j.runtime.installer.Installer.getScreenExecutor(Installer.java:88)
       at com.install4j.runtime.installer.Installer.runInProcess(Installer.java:57)
       at com.install4j.runtime.installer.Installer.main(Installer.java:45)
       at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
       at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
       at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
       at java.base/java.lang.reflect.Method.invoke(Method.java:566)
       at com.exe4j.runtime.LauncherEngine.launch(LauncherEngine.java:85)
       at com.install4j.runtime.launcher.UnixLauncher.main(UnixLauncher.java:62)