> p_mb
Simple feature collection with 52 features and 0 fields (with 9 geometries empty)
geometry type:  GEOMETRY
dimension:      XY
bbox:           xmin: 585001.4 ymin: 211001.9 xmax: 609998.1 ymax: 236998.6
epsg (SRID):    NA
proj4string:    NA
First 10 features:
                         geometry
1  POLYGON ((591051.4 211855, ...
2                   POLYGON EMPTY
3  POLYGON ((588206.4 212570, ...
4  MULTIPOLYGON (((585004.6 21...
5  POLYGON ((600614.5 216112.9...
6  POLYGON ((603033.8 215125.2...
7  MULTIPOLYGON (((608168.4 21...
8  POLYGON ((600009.4 212984, ...
9  POLYGON ((603203.4 217336, ...
10 POLYGON ((585519.4 217546, ...

> p_pol <- sf::st_cast(p_mb0, "POLYGON")
There were 23 warnings (use warnings() to see them)
> warnings()
1: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
2: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
3: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
4: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
5: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
6: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
7: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
8: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
9: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
10: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
11: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
12: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
13: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
14: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
15: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
16: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
17: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
18: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
19: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
20: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
21: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
22: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only
23: In st_cast.MULTIPOLYGON(X[[i]], ...) : polygon from first part only

> spl_mbuf <- as(p_pol, "Spatial")
Error: length(srl) > 0 is not TRUE