fix(picker): nil check manager on toggle_selection (#3283)

Probably due to some async race condition, when typing fast, an error "attempt to index field 'manager' (a boolean value)" can be thrown if accidentally pressed <Tab> or <C-i> 

This is a temp fix for that.

Author: xudyang1

lua/telescope/pickers.lua

@@ -957,7 +957,7 @@ end
 --- Also updates the highlighting for the given entry
 ---@param row number: the number of the chosen row
 function Picker:toggle_selection(row)
-  local entry = self.manager:get_entry(self:get_index(row))
+  local entry = self.manager and self.manager:get_entry(self:get_index(row))
   if entry == nil then
     return
   end