IcedTea-Web app fails to start due to missing XDownloadService.getDefaultProgressWindow() implementation

Issue

The IcedTea-Web app fails to start, and the following is in the IcedTea-Web debug logging:

[ERROR_DEBUG][Wed Apr 07 11:51:06 CEST 2021][net.sourceforge.jnlp.services.ServiceUtil$PrivilegedHandler.invoke(ServiceUtil.java:187)] NETX Thread# ef7f6ab, name Object Browser Client call privileged method: getDefaultProgressWindow
[ERROR_DEBUG][Wed Apr 07 11:51:06 CEST 2021][net.sourceforge.jnlp.services.ServiceUtil$PrivilegedHandler.invoke(ServiceUtil.java:205)] NETX Thread# ef7f6ab, name Object Browser Client         result: null
[MESSAGE_DEBUG][Wed Apr 07 11:51:06 CEST 2021][net.sourceforge.jnlp.runtime.JNLPRuntime.markNetxStopped(JNLPRuntime.java:866)] NETX Thread# 73a28541, name JNLPRuntimeShutdownHookThread Release shared lock on C:\Users\USER\AppData\Local\Temp\117\hse8fe\netx\locks\netx_running
[MESSAGE_DEBUG][Wed Apr 07 11:51:06 CEST 2021][net.sourceforge.jnlp.cache.CacheUtil.okToClearCache(CacheUtil.java:394)] NETX Thread# 73a28541, name JNLPRuntimeShutdownHookThread No other instances of javaws are running