Try
{
    RegWrite, REG_SZ, HKCR, .test
    RootKey=HKCR
    Subkey=
}
Catch e
{
    RootKey=HKCU
    Subkey=Software\Classes\
}
Finally
{
    RegDelete, HKCR, .test
}
Try
{
    RegRead, FileType, %RootKey%\%Subkey%.ahk
}
Catch e
{
    FileType := "AutoHotkeyScript"
}
FileSelectFile, Editor_Path, 1, , Choose An Editor..., Executalbe Files(*.exe)
MsgBox %RootKey%\%Subkey%%FileType%\Shell\Edit\Command
RegWrite, REG_SZ, %RootKey%\%Subkey%%FileType%\Shell\Edit\Command, , "%Editor_Path%" "`%1"