Try this: Select the XML file in finder (so it's highlighted in blue). Then click File (at the top of the screen). In the box that opens, click Get Info. (The shortcut to do all this is pressing Command and I at the same time). A box will open. Click on the blue arrows next to "Open with:". Then click on TextEdit.app. Then click "Change All...". Another box will open. Click "Continue". That should do it.
The xml is designed to be read by other NLE software. Usually it opens in a browser if you just click on it. Unless you need to edit the xml outside FCP there is no need to 'open' it from the saved file. If you are transfering your edit to Resolve for example then open Resolve, >import timeline>select the xml file.