Deal with missing minsowdate or maxsowdate for crop

src/farm/farm.jl

@@ -64,7 +64,11 @@ function stepagent!(farmer::BasicFarmer, model::SimulationModel)
                 @sow("no growth")
                 cycle!(farmer.croprotations[f]) # advance the crop rotation
                 nextcrop = model.crops[farmer.croprotations[f][1]]
-                farmer.sowdates[f] = @rand(nextcrop.minsowdate:nextcrop.maxsowdate)
+                if ! (ismissing(nextcrop.minsowdate) || ismissing(nextcrop.maxsowdate))
+                    farmer.sowdates[f] = @rand(nextcrop.minsowdate:nextcrop.maxsowdate)
+                else
+                    @warn "minsowdate and/or maxsowdate is `missing` for crop \"$(ctype.name)\", not assigning sowdates"
+                end
             end
         elseif cropname(field) == "no growth" && model.date == farmer.sowdates[f]
             # if a field has been harvested, check if the next crop can be sown