lldb type synthetic python function execute failed when variable is shown in Watches View

I've write a lldb type synthetic for a complex struct,

it will use CreateValueFromExpression to create some child value and it works fine to transform the variable shown in Variables View,

but when I add the same variable to Watches View, it won't show the expected result and poduce an error:

the python snippet is:

if self.tag == JS_TAG_MAP["JS_TAG_FLOAT64"]:
    fv = self.valobj.GetChildMemberWithName("u").GetChildMemberWithName("float64").GetData().GetDouble(e, 0)
    self.propList.append({"name": "float", "value": self.valobj.CreateValueFromExpression("float", str(fv))})
    desc = str(fv)
elif self.tag == JS_TAG_MAP["JS_TAG_INT"]:
    fv = self.valobj.GetChildMemberWithName("u").GetChildMemberWithName("int32").GetValueAsSigned()
    self.propList.append({"name": "int", "value": self.valobj.CreateValueFromExpression("int", str(fv))})
    desc = str(fv)
elif self.tag == JS_TAG_MAP["JS_TAG_STRING"]:
    svexp = self.valobj.CreateValueFromExpression("str", 'JS_ToCStringLen2(0,0,' + self.valobj.name + ',0)')
    self.propList.append({"name": "str", "value": svexp})
    desc = valobj.frame.GetThread().GetProcess().ReadCStringFromMemory(svexp.GetValueAsUnsigned(), 0xffffff, e)