Fix Exception thrown on farmOS 3.1 due to harvest_days field not bein…

…g available on plant types #33

src/Form/CropPlanAddPlantingForm.php

@@ -285,7 +285,7 @@ public function plantingDefaultValues($plant = NULL) {
       }
 
       // Populate harvest_days from the plant_type.
-      if ($plant_type->get('harvest_days')->value) {
+      if ($plant_type->hasField('harvest_days') && $plant_type->get('harvest_days')->value) {
         $values['harvest_days'] = $plant_type->get('harvest_days')->value;
       }
     }