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Fix T77047: Dyntopo Sample detail size on hidden mesh causes crash

The `Toolbar` and `Sidebar` hide the corresponding panel
`VIEW3D_PT_sculpt_dyntopo` by polling for context.sculpt_object and
context.tool_settings.sculpt. In the Active Tool in the Properties
Editor this poll does not return False though, thus the
sample_detail_size is possible from there.

Second security check (the operator poll `SCULPT_mode_poll`) checks the
active object -- that is still valid even if hidden, so we are allowed
to execute the operator. However the active object becomes NULL once the
area is switched in `sample_detail()` -- see `CTX_wm_area_set`), leading
to the crash.

Dont think there is a quick and easy way to do this in the poll from the
Properties Editor, so just check for a valid active abject in the
operator and return OPERATOR_CANCELLED if we dont have it.

Maniphest Tasks: T77047

Differential Revision: https://developer.blender.org/D7832