StorageMap can't be written in static variables

[chenzhitong]

I found that if StorageMap is written in a global variables, the execution will report an error.
If the StorageMap is written in a variable in the method, it is executed correctly.

[Features(ContractFeatures.HasStorage)]
public class Contract1 : SmartContract
{
    private static readonly StorageMap Data = Storage.CurrentContext.CreateMap("data");

    public static void Put(string message)
    {
        Data.Put(message, Blockchain.GetHeight());
    }

    public static BigInteger Get(string msg)
    {
        return Data.Get(msg)?.ToBigInteger() ?? 0;
    }

    public static void Put2(string message)
    {
        var Data2 = Storage.CurrentContext.CreateMap("data");
        Data2.Put(message, Blockchain.GetHeight());
    }

    public static BigInteger Get2(string msg)
    {
        var Data2 = Storage.CurrentContext.CreateMap("data");
        return Data2.Get(msg)?.ToBigInteger() ?? 0;
    }
}

The put and get methods will fail to execute, but the put2 and get2 methods will execute successfully.

This bug will appear in both Neo2 and Neo3

The picture below is the execution result:

StorageMap 如果写在全局变量（静态变量）中，会执行出错，但写在方法里面就能正常执行，从Neo2到现在一直存在的Bug。为此我还补充了文档。但我觉得有坑的话应该先把坑填上，而不是叫所有人都绕过去走。

[shargon]

Could you take a look @lightszero ?

[lightszero]

I think this is about #237

[ShawnYun]

I will check it, but according to the above mentioned, this bug will appear in both Neo2 and Neo3.