Fix bug with container on Windows.

This fix the bug from:
https://github.com/ArthurSonzogni/FTXUI/pull/11

About:
~~~
Bug: Focus handling not working

in the examples (e.g. checkbox.cpp) I can toggle the individual
checkboxes but I cannot move between items, I tried to understand the
focus implementation but am I unsure which keypresses would move focus
between different components
~~~

include/ftxui/component/container.hpp

@@ -37,7 +37,7 @@ class Container : public Component {
   RenderHandler render_handler_;
 
   int selected_ = 0;
-  int* selector_ = &selected_;
+  int* selector_ = nullptr;
 };
 
 }  // namespace ftxui