FlexForm blocks edit of systemmessage

[wgevaert]

Cannot edit message that does not have a page

Steps to reproduce:
0. Install CSP Basis

1. Set $wgContentlang = 'nl'
2. Try to edit MediaWiki:Ws-header/EN
3. Click "preview" ("Bewerking ter controle bekijken")
   You get this error:

[212f27016ca2e8d8d3d953f0] /MediaWiki:Ws-header/EN?action=submit TypeError from line 59 of /wiki/public_html/extensions/FlexForm/src/Core/Rights.php: Argument 1 passed to FlexForm\Core\Rights::isThereAFlexFormInThePageContent() must be an instance of WikiPage, null given, called in /wiki/public_html/extensions/FlexForm/src/Core/Rights.php on line 167

Backtrace:

#0 /wiki/public_html/extensions/FlexForm/src/Core/Rights.php(167): FlexForm\Core\Rights::isThereAFlexFormInThePageContent()
#1 /wiki/public_html/includes/HookContainer/HookContainer.php(329): FlexForm\Core\Rights::disableActions()
#2 /wiki/public_html/includes/HookContainer/HookContainer.php(132): MediaWiki\HookContainer\HookContainer->callLegacyHook()
#3 /wiki/public_html/includes/HookContainer/HookRunner.php(1990): MediaWiki\HookContainer\HookContainer->run()
#4 /wiki/public_html/includes/Permissions/PermissionManager.php(451): MediaWiki\HookContainer\HookRunner->onGetUserPermissionsErrors()
#5 /wiki/public_html/includes/Permissions/PermissionManager.php(409): MediaWiki\Permissions\PermissionManager->checkPermissionHooks()
#6 /wiki/public_html/includes/Permissions/PermissionManager.php(243): MediaWiki\Permissions\PermissionManager->getPermissionErrorsInternal()
#7 /wiki/public_html/extensions/WSSlots/src/Scribunto/ScribuntoLuaLibrary.php(190): MediaWiki\Permissions\PermissionManager->userCan()
#8 /wiki/public_html/extensions/WSSlots/src/Scribunto/ScribuntoLuaLibrary.php(116): WSSlots\Scribunto\ScribuntoLuaLibrary->userCan()
#9 /wiki/public_html/extensions/Scribunto/includes/engines/LuaSandbox/LuaSandboxCallback.php(26): WSSlots\Scribunto\ScribuntoLuaLibrary->slotData()
#10 [internal function]: Scribunto_LuaSandboxCallback->__call()
#11 /wiki/public_html/extensions/Scribunto/includes/engines/LuaSandbox/LuaSandboxInterpreter.php(113): LuaSandboxFunction->call()
#12 /wiki/public_html/extensions/Scribunto/includes/engines/LuaCommon/LuaEngine.php(290): Scribunto_LuaSandboxInterpreter->callFunction()
#13 /wiki/public_html/extensions/Scribunto/includes/engines/LuaCommon/LuaModule.php(68): Scribunto_LuaEngine->executeFunctionChunk()
#14 /wiki/public_html/extensions/Scribunto/includes/common/Hooks.php(140): Scribunto_LuaModule->invoke()
#15 /wiki/public_html/includes/parser/Parser.php(3339): ScribuntoHooks::invokeHook()
#16 /wiki/public_html/includes/parser/Parser.php(3046): Parser->callParserFunction()
#17 /wiki/public_html/includes/parser/PPFrame_Hash.php(263): Parser->braceSubstitution()
#18 /wiki/public_html/includes/parser/Parser.php(2962): PPFrame_Hash->expand()
#19 /wiki/public_html/includes/parser/PPFrame_Hash.php(263): Parser->braceSubstitution()
#20 /wiki/public_html/includes/parser/PPTemplateFrame_Hash.php(89): PPFrame_Hash->expand()
#21 /wiki/public_html/includes/parser/Parser.php(3221): PPTemplateFrame_Hash->cachedExpand()
#22 /wiki/public_html/includes/parser/PPFrame_Hash.php(263): Parser->braceSubstitution()
#23 /wiki/public_html/includes/parser/Parser.php(2886): PPFrame_Hash->expand()
#24 /wiki/public_html/includes/parser/Parser.php(1557): Parser->replaceVariables()
#25 /wiki/public_html/includes/parser/Parser.php(652): Parser->internalParse()
#26 /wiki/public_html/includes/content/WikitextContent.php(374): Parser->parse()
#27 /wiki/public_html/includes/content/AbstractContent.php(590): WikitextContent->fillParserOutput()
#28 /wiki/public_html/includes/EditPage.php(4286): AbstractContent->getParserOutput()
#29 /wiki/public_html/includes/EditPage.php(4191): EditPage->doPreviewParse()
#30 /wiki/public_html/includes/EditPage.php(2969): EditPage->getPreviewText()
#31 /wiki/public_html/includes/EditPage.php(701): EditPage->showEditForm()
#32 /wiki/public_html/includes/actions/EditAction.php(71): EditPage->edit()
#33 /wiki/public_html/includes/actions/SubmitAction.php(38): EditAction->show()
#34 /wiki/public_html/includes/MediaWiki.php(531): SubmitAction->show()
#35 /wiki/public_html/includes/MediaWiki.php(313): MediaWiki->performAction()
#36 /wiki/public_html/includes/MediaWiki.php(947): MediaWiki->performRequest()
#37 /wiki/public_html/includes/MediaWiki.php(547): MediaWiki->main()
#38 /wiki/public_html/index.php(53): MediaWiki->run()
#39 /wiki/public_html/index.php(46): wfIndexMain()
#40 {main}