Python class preview fails in GH when no Rhino geometry present in viewport

I created a custom class in Python to contain some geometry and other information. I implemented DrawViewportMeshes, DrawViewportWires and get_ClippingBox to be able to preview the geometry contained in instances of the class. I then created a number of GH nodes operating on these instances. The results are nicely visualized in Rhino, but only if the viewport also includes some Rhino geometry. When I pan the viewport and the Rhino geometry disappears from the viewport, everything suddenly disappears even though it is still within the viewport. As soon as the Rhino geometry comes back in view everything becomes visible again. Below is the code I’m using, anybbody have any idea what I’m doing wrong? It happens in both Rhino 6 and 7 (on mac).

class RhinoShape(gh.IGH_BakeAwareData, gh.IGH_PreviewData):

def get_Boundingbox(self):
if self.bbox is not None:
return self.bbox

        box = None
        for geom in self.getGeometry():
                tempBox = geom.GetBoundingBox(rg.Plane.WorldXY)
                if type(geom) != rg.Point3d and type(geom) != type([]):
                    tempBox =  geom.get_Boundingbox()
                elif type(geom[0]) == type(TextGoo):
                    tempBox =  geom[0].get_Boundingbox()
                    tempBox = None
            if box == None:
                box = tempBox
            elif tempBox != None:
                box = rg.BoundingBox.Union(box,tempBox)
        self.bbox = box.GetCorners()
            self.bbox = rhinoscriptsyntax.BoundingBox(geometries)
            self.bbox = None
    return self.bbox
def get_ClippingBox(self):
    if self.bbox is None:
    return self.bbox

def DrawViewportWires(self, args): #args of type GH_PreviewWireArgs
    pipeline = args.Pipeline
    for index, item in enumerate(self.getGeometry()):
        itype = type(item)
        colorValue = self.colors[index]
        if colorValue is None:
            colorValue = args.Color
        if itype is rg.Point:
            pipeline.DrawPoint(item.Location, Rhino.Display.PointStyle.X, 5, colorValue)
        elif itype is rg.LineCurve or itype is rg.Curve:
            pipeline.DrawCurve(item, colorValue, args.Thickness)
        elif itype is rg.Brep:
            pipeline.DrawBrepWires(item, colorValue)
        elif itype is rg.Circle:
            pipeline.DrawCircle(item, colorValue, args.Thickness)
        elif itype is Rhino.Display.Text3d:
            pipeline.Draw3dText(item, args.Color)
            print("Not addressed: " + str(itype))
def DrawViewportMeshes(self, args): #args of type GH_PreviewMeshArgs
    pipeline = args.Pipeline
    for index, item in enumerate(self.getGeometry()):
        if type(item) is rg.Brep:
            colorValue = self.colors[index]
            if colorValue is not None:
                material = Rhino.Display.DisplayMaterial(colorValue)
                material = args.Material
            pipeline.DrawBrepShaded(item, material)

def BakeGeometry(self, doc, att, id):