Is it possible to restart an application using AppleScript?
An alternative method is to use Safari's built-in "Reopen All Windows from Last Session" feature:

tell application "Safari"
    quit
end tell

delay 2 -- Wait for Safari to close

tell application "Safari" to activate
tell application "System Events"
    tell process "Safari"
        click menu item "Reopen All Windows From Last Session" of menu "History" of menu bar 1
    end tell
end tell