Prevent crash on exit in scroll mode (#3171)

frontend/apps/reader/modules/readerrolling.lua

@@ -140,11 +140,22 @@ function ReaderRolling:onReadSettings(config)
     self.inverse_reading_order = config:readSetting("inverse_reading_order") or false
 end
 
+-- in scroll mode percent_finished must be save before close document
+-- we cannot do it in onSaveSettings() because getLastPercent() uses self.ui.document
+function ReaderRolling:onCloseDocument()
+    self.ui.doc_settings:saveSetting("percent_finished", self:getLastPercent())
+end
+
 function ReaderRolling:onSaveSettings()
     -- remove last_percent config since its deprecated
     self.ui.doc_settings:saveSetting("last_percent", nil)
     self.ui.doc_settings:saveSetting("last_xpointer", self.xpointer)
-    self.ui.doc_settings:saveSetting("percent_finished", self:getLastPercent())
+    -- in scrolling mode, the document may already be closed,
+    -- so we have to check the condition to avoid crash function self:getLastPercent()
+    -- that uses self.ui.document
+    if self.ui.document then
+        self.ui.doc_settings:saveSetting("percent_finished", self:getLastPercent())
+    end
     self.ui.doc_settings:saveSetting("show_overlap_enable", self.show_overlap_enable)
     self.ui.doc_settings:saveSetting("inverse_reading_order", self.inverse_reading_order)
 end