When a page was modified externally and is read by a plugin via WikiEngine.getPage(..) during intialization a NPE is thrown

Our plugin extends BasicPageFilter and implements WikiPlugin and WikiEventListener. During the initialisation of the wikisystem we are getting the source of a specific page. (some settings are stored there).
This works quite well most of the time. But when this page was externally modified (by editor...) the call to getPage(..) results in a null result. I've traced it down to the location of it happening: in the class WikiEngine, the methoed

private String textToHTML( WikiContext context, String pagedata, StringTransmutator localLinkHook,
StringTransmutator extLinkHook, StringTransmutator attLinkHook, boolean parseAccessRules,
boolean justParse )

calls
pagedata = m_filterManager.doPreTranslateFiltering( context, pagedata );
in line 1615

at this point the m_filterManager isn't initialized yet, therefore resulting in an NPE.