Avoid crash when back_leveling_delay is None.

issue3195
review785002
(transplanted from fcd869cb3d0c35b2aa5907ef5ad4fe0202b8a8d8)

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extra : transplant_source : %FC%D8i%CB%3D%0C5%B2%AAY%07%EFZ%D4%FE%02%02%B8%A8%D8