Due to an unintentional break in backwards compatibility in MODX 2.5.0-rc1, you might find the dashboard widget of SimpleCart throwing a fatal error.
This was first reported by @pixelfanatiker at https://github.com/modxcms/revolution/issues/12895, and I’ve just sent a proposed patch to resolve this in the next release of 2.5: https://github.com/modxcms/revolution/pull/12897
If you’re running into the issue, you can try applying the above patch manually or temporarily removing the widget from your dashboard. It might also affect other extras.