Got error NestedLoopedMicroflow when trying to update many-to-many association in verions 2.5.1

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I converted my project from 2.4.4.1 to 2.5.1 and got the problem with 1 microflow which trying to update many-to-many association between 2 entities. Is this bug or something I can solve via modifying MF? The scenario is as below. 1 Batch contains many production orders. 1 Batch generates many scraps. Each scraps should be able to refer to all production orders in each Batch. My MF works fine in 2.4.4.1 but in 2.5.1 I got below error no matter I looped scraps first and then looped production orders inside, get the existing production orders from scrap, union existing ones with the looped one, and update to scrap. Or looped production orders first. Error message : 2010-09-28 10:55:39.000 MicroflowEngine Exception occurred in microflow 'Production.CalcBatchScrap.nested.1562760.nested.1562768' for activity 'Union'', all database changes executed by this microflow were rolled back 2010-09-28 10:55:39.000 MicroflowEngine Exception occurred in microflow 'Production.CalcBatchScrap.nested.1562760' for activity 'NestedLoopedMicroflow'', all database changes executed by this microflow were rolled back 2010-09-28 10:55:39.000 MicroflowEngine Exception occurred in microflow 'Production.CalcBatchScrap' for activity 'NestedLoopedMicroflow'', all database changes executed by this microflow were rolled back 2010-09-28 10:55:39.000 MicroflowEngine Exception occurred in microflow 'Inventory.CalcAvailableMaterialUsageFromOutgoing' for activity 'Calc batch scrap'', all database changes executed by this microflow were rolled back 2010-09-28 10:55:39.000 ActionManager Exception occurred in action 'EventExtendedAction :: (...) 2010-09-28 10:55:39.000 Connector com.mendix.core.CoreException: com.mendix.core.CoreException: com.mendix.core.CoreException: com.mendix.core.CoreException: java.lang.NullPointerException
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Probably one of the arguments provided to the union is 'empty' (that is not: the list contains zero elements, but: that the list is NULL) or contains an 'empty' element. A union of a list and a NULL (empty) list will throw this exception, at least the last time i tried.

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