Fix crash when toggling auto-sync without a document open

Guard nil doc_settings in getBookhoardUUID and skip getProgress
call in toggleAutoSync when no document is loaded.

main.lua

@@ -471,7 +471,7 @@ function Bookhoard:toggleAutoSync()
     self:registerEvents()
     G_reader_settings:saveSetting(self.settings_key, self.settings)
 
-    if self.settings.auto_sync then
+    if self.settings.auto_sync and self.ui.doc_settings then
         self:getProgress(true, true)
     end
 end
@@ -602,6 +602,7 @@ function Bookhoard:getFileSHA256()
 end
 
 function Bookhoard:getBookhoardUUID()
+    if not self.ui.doc_settings then return nil end
     return self.ui.doc_settings:readSetting("bookhoard_uuid")
 end