This fixes a bunch of errors I was getting when switching away from the
avante sidebar and then back. Sometimes the sidebar would lose track of
the various windows, and you'd get errors like this:
```
|| stack traceback:
|| ...local/share/nvim/lazy/avante.nvim/lua/avante/sidebar.lua:2350: in function <...local/share/nvim/lazy/avante.nvim/lua/avante/sidebar.lua:2349>
|| Error detected while processing BufLeave Autocommands for "<buffer=294>":
|| Error executing lua callback: ...local/share/nvim/lazy/avante.nvim/lua/avante/sidebar.lua:2398: attempt to index field 'selected_files_container' (a nil value)
```
Or this one:
```
|| stack traceback:
|| ...local/share/nvim/lazy/avante.nvim/lua/avante/sidebar.lua:2350: in function <...local/share/nvim/lazy/avante.nvim/lua/avante/sidebar.lua:2349>
|| Error detected while processing BufLeave Autocommands for "<buffer=294>":
|| Error executing lua callback: ...local/share/nvim/lazy/avante.nvim/lua/avante/sidebar.lua:2398: attempt to index field 'selected_files_container' (a nil value)
```
This change does the following:
1. Proper order of cleanup in `reset()`
2. Proper cleanup in `create_selected_files_container()`
0bfbd39ebc