CURA-1223: Checking whether the bounding box is correct

Adds an additional check, like suggested by @awhiemstra.

Contributes to CURA-1223

plugins/3MFReader/ThreeMFReader.py

@@ -124,7 +124,8 @@ class ThreeMFReader(MeshReader):
             Logger.log("e", "exception occured in 3mf reader: %s", e)
 
         try: # Selftest - There might be more functions that should fail 
-            result.getBoundingBox()
+            boundingBox = result.getBoundingBox()
+            boundingBox.isValid()
         except:
             message = Message(catalog.i18nc("@info:status", "Your 3MF file seems to be broken. Please visit https://modelrepair.azurewebsites.net/ and try to repair your model!"), lifetime = 0)
             message.show()