Exceptions in the log after deleting a subwiki

Using the xwiki-platform-distribution-flavor-jetty-hsqldb-9.11.7.zip, I created the main wiki using the demo flavor and a couple of subwikis called test1 and test2 with the "XWiki Standard Flavor - Sub Wiki". Then I deleted the subwikis and I now a get large chunk of log messages every hour (see attached xwiki.log). It starts with this:

2018-08-14 14:00:00,001 [DefaultQuartzScheduler_Worker-4] WARN o.h.u.JDBCExceptionReporter - SQL Error: -4850, SQLState: 3F000
2018-08-14 14:00:00,002 [DefaultQuartzScheduler_Worker-4] ERROR o.h.u.JDBCExceptionReporter - invalid schema name: TEST1
2018-08-14 14:00:00,002 [DefaultQuartzScheduler_Worker-5] WARN o.h.u.JDBCExceptionReporter - SQL Error: -4850, SQLState: 3F000
2018-08-14 14:00:00,003 [DefaultQuartzScheduler_Worker-5] ERROR o.h.u.JDBCExceptionReporter - invalid schema name: TEST2
2018-08-14 14:00:00,009 [DefaultQuartzScheduler_Worker-5] ERROR PreferencesConfigurationSource - Failed to access configuration property
com.xpn.xwiki.XWikiException: Error number 3202 in 3: Exception while reading document [test2:Scheduler.WebPreferences]
at com.xpn.xwiki.store.XWikiHibernateStore.loadXWikiDoc(XWikiHibernateStore.java:1008)
at com.xpn.xwiki.store.XWikiCacheStore.loadXWikiDoc(XWikiCacheStore.java:280)
at com.xpn.xwiki.XWiki.getDocument(XWiki.java:1854)
at com.xpn.xwiki.XWiki.getDocument(XWiki.java:1914)
at org.xwiki.configuration.internal.AbstractXWikiPreferencesConfigurationSource.getBaseObject(AbstractXWikiPreferencesConfigurationSource.java:86)
at org.xwiki.configuration.internal.AbstractXWikiPreferencesConfigurationSource.getBaseProperty(AbstractXWikiPreferencesConfigurationSource.java:139)
at org.xwiki.configuration.internal.AbstractXWikiPreferencesConfigurationSource.getBaseProperty(AbstractXWikiPreferencesConfigurationSource.java:156)
at org.xwiki.configuration.internal.AbstractDocumentConfigurationSource.getPropertyValue(AbstractDocumentConfigurationSource.java:313)
at org.xwiki.configuration.internal.AbstractDocumentConfigurationSource.containsKey(AbstractDocumentConfigurationSource.java:192)
at org.xwiki.configuration.internal.AbstractSpacesConfigurationSource$SpaceConfigurationSource.containsKey(AbstractSpacesConfigurationSource.java:172)
at org.xwiki.configuration.internal.AbstractCompositeConfigurationSource.containsKey(AbstractCompositeConfigurationSource.java:45)
at org.xwiki.configuration.internal.AbstractCompositeConfigurationSource.containsKey(AbstractCompositeConfigurationSource.java:45)
at org.xwiki.configuration.internal.AbstractCompositeConfigurationSource.getProperty(AbstractCompositeConfigurationSource.java:95)
at org.xwiki.url.internal.DefaultURLConfiguration.getURLFormatId(DefaultURLConfiguration.java:55)
at org.xwiki.url.internal.URLExecutionContextInitializer.initialize(URLExecutionContextInitializer.java:55)
at org.xwiki.context.internal.DefaultExecutionContextManager.runInitializers(DefaultExecutionContextManager.java:115)
at org.xwiki.context.internal.DefaultExecutionContextManager.initialize(DefaultExecutionContextManager.java:103)
at com.xpn.xwiki.plugin.scheduler.AbstractJob.execute(AbstractJob.java:69)
at org.quartz.core.JobRunShell.run(JobRunShell.java:202)
at org.quartz.simpl.SimpleThreadPool$WorkerThread.run(SimpleThreadPool.java:573)