Crash when switching to a non-existent project

[jubnzv]

When we try to switch to a non-existent project, the plugin crashes. I think it would be nice to handle such cases and display a warning message to the user.

To reproduce:

1. Open telescope-project and enter the name of some non-existent project:
2. Press Enter to see the crash:

E5108: Error executing lua ...oject.nvim/lua/telescope/_extensions/project/actions.lua:18: attempt to index a nil value                                                                                              
stack traceback:                                                                                                                                                                                                     
        ...oject.nvim/lua/telescope/_extensions/project/actions.lua:18: in function 'get_selected_path'                                                                                                              
        ...oject.nvim/lua/telescope/_extensions/project/actions.lua:101: in function 'run_replace_or_original'                                                                                                       
        ...nvim/plugged/telescope.nvim/lua/telescope/actions/mt.lua:30: in function 'find_project_files'                                                                                                             
        ...-project.nvim/lua/telescope/_extensions/project/main.lua:73: in function 'run_replace_or_original'                                                                                                        
        ...nvim/plugged/telescope.nvim/lua/telescope/actions/mt.lua:30: in function 'key_func'                                                                                                                       
        ...e/nvim/plugged/telescope.nvim/lua/telescope/mappings.lua:235: in function 'execute_keymap'                                                                                                                
        [string ":lua"]:1: in main chunk 

[jubnzv]

It seems, that we could modify the M.get_selected_title and M.get_selected_path functions from actions.lua to make them return a nil value if there is no such project.