This kind of exceptions is one of the easiest to debug and fix. If happens, when you try to dereference some variable/member of some reference type by using one of the instance member of this type. Any such operations requires the reference to be non-null; If it's a null, this exception is thrown. In your case,
should be null. Check up the index value "Constr" — it could be misspelled, or a connection string under this name is not found in the configuration.