系统环境：7.1 更新过PT120306补丁。
Nullable object must have a value.
[{"operation":"BOS_ConvertOpForm.ButtonClick"}]
发生时间： 11:46:18
错误编号： eb32bb3840f845beb8a782b9998310c8
错误信息： Nullable object must have a value.
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调用堆栈：
Server stack trace:
at System.Nullable`1.get_Value()
at Kingdee.K3.SCM.App.Sal.ServicePlugIn.SaleOrderPur.IsUseTaxCombination()
at Kingdee.K3.SCM.App.Sal.ServicePlugIn.SaleOrderPur.OnAfterFieldMapping(AfterFieldMappingEventArgs e)
at Kingdee.BOS.Core.Metadata.ConvertElement.PlugIn.ConvertPlugInProxy.FireOnAfterFieldMapping(AfterFieldMappingEventArgs e)
at Kingdee.BOS.App.Core.Convertible.DefaultPolicy.FieldMappingAction.Do()
at Kingdee.BOS.Core.BusinessPolicyModel.BusinessPolicyEngine.Do(OperateOption context)
at Kingdee.BOS.App.Core.Convertible.ConvertEngine.OnCreating(PushArgs pushArgs, OperateOption option)
at Kingdee.BOS.App.Core.ConvertService.Push(Context ctx, PushArgs serviceArgs, OperateOption option, Boolean validPermission)
Exception r
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