Changing the colour scheme may result in this exception:

java.lang.NullPointerException
at com.opensymphony.oscache.base.algorithm.AbstractConcurrentReadCache.getTableForReading(AbstractConcurrentReadCache.java:940)
at com.opensymphony.oscache.base.algorithm.AbstractConcurrentReadCache$HashIterator.(AbstractConcurrentReadCache.java:1946)
at com.opensymphony.oscache.base.algorithm.AbstractConcurrentReadCache$KeyIterator.(AbstractConcurrentReadCache.java:2018)
at com.opensymphony.oscache.base.algorithm.AbstractConcurrentReadCache$2.iterator(AbstractConcurrentReadCache.java:779)
at com.opensymphony.oscache.base.Cache.flushPattern(Cache.java:487)
at com.opensymphony.oscache.base.Cache.flushPattern(Cache.java:470)
at com.atlassian.confluence.admin.actions.lookandfeel.AbstractLookAndFeelAction.flushOSCacheFilter(AbstractLookAndFeelAction.java:148)
at com.atlassian.confluence.admin.actions.lookandfeel.ColourSchemeAction.execute(ColourSchemeAction.java:29)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:168)
at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:35)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:165)
at com.opensymphony.xwork.interceptor.DefaultWorkflowInterceptor.intercept(DefaultWorkflowInterceptor.java:55)
at com.atlassian.confluence.core.ConfluenceWorkflowInterceptor.intercept(ConfluenceWorkflowInterceptor.java:33)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:165)
at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:35)
at com.atlassian.confluence.core.ConfluenceValidationInterceptor.intercept(ConfluenceValidationInterceptor.java:16)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:165)
at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:35)