How to fix compilation for web?

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Unity 6000.2.5f ArgumentNullException: Value cannot be null. Parameter name: type

VariableManager

        public void OnAfterDeserialize()
        {
            Variables.Clear();
            for (int i = 0; i < VariableValues.Count; i++)
                Variables.Add(SerializedData.Deserialize(VariableValues[i]) as Variable);
        }