Fix #1645. Fix bug where you could accidentally have a layer set with zero thickness which caused object creation to fail.

src/blenderbim/blenderbim/bim/module/model/slab.py

@@ -249,7 +249,7 @@ class DumbSlabGenerator:
                 if material.is_a("IfcMaterialLayerSet"):
                     thicknesses = [l.LayerThickness for l in material.MaterialLayers]
                     break
-        if not thicknesses:
+        if not sum(thicknesses):
             return
 
         self.collection = bpy.context.view_layer.active_layer_collection.collection

src/blenderbim/blenderbim/bim/module/model/wall.py

@@ -643,7 +643,7 @@ class DumbWallGenerator:
                 if material.is_a("IfcMaterialLayerSet"):
                     thicknesses = [l.LayerThickness for l in material.MaterialLayers]
                     break
-        if not thicknesses:
+        if not sum(thicknesses):
             return
 
         self.collection = bpy.context.view_layer.active_layer_collection.collection

src/ifcopenshell-python/ifcopenshell/api/material/add_layer.py

@@ -9,7 +9,7 @@ class Usecase:
         layers = list(self.settings["layer_set"].MaterialLayers or [])
         layer = self.file.create_entity("IfcMaterialLayer", **{
             "Material": self.settings["material"],
-            "LayerThickness": 0.
+            "LayerThickness": 1.
         })
         layers.append(layer)
         self.settings["layer_set"].MaterialLayers = layers