ref(Popup): set Popup.window to nil after :close()

Before this commit, Popup.window would only be set to `nil` if there was
a valid window found. It is proposed that `nil` shuold be set especially
if there is no valid window found, as an invalid window value shouldn't
be kept around.

lua/libmodal/src/collections/Popup.lua

@@ -65,9 +65,10 @@ local _metaPopup = require('libmodal/src/classes').new(Popup.TYPE)
 function _metaPopup:close(keepBuffer)
 	if valid(self.window) then
 		api.nvim_win_close(self.window, false)
-		self.window = nil
 	end
 
+	self.window = nil
+
 	if not keepBuffer then
 		self.buffer = nil
 		self._inputChars = nil