Reproduction steps:
1) Open attached project
2) Open "rep" scene
3) Make some changes (change position or color, doesn't matter) to prefab (Image or Image (1) insice Canvas > Scroll view > Viewport > Content)
4) Press Ctrl+Z

-- Notice, "MissingReferenceException: The object of type 'Material' has been destroyed but you are still trying to access it." is being thrown

Regression introduced in 5.6.0b2

Reproduced with:
5.6.0b5, 5.6.0b2,

Unable to reproduce with:
5.5.1p1, 5.6.0b1