Safari may still be open. Closing the browser window doesn't actually quit the application. When prompted to quit Safari, launch Activity Monitor from /Applications/Utilities. Look for Safari, select it and then click 'Quit Process'.
I'm having this problem too. Safari is NOT open. The installer is erroneously reporting that Safari is running when it isn't. It's not in the Activity Monitor; it's not in the program switcher, and the dock icon doesn't show it as running. I don't know what criteria the installer is using to reach the conclusion that Safari is running, but it's WRONG!