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https://issues.apache.org/jira/browse/FLEX-33311?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15426488#comment-15426488
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Mihai Chira commented on FLEX-33311:
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[~riastar], thank you for reporting this and adding sample code. It was very
useful, and with your help (and Alex's) the bug is now fixed.
> Null pointer fatal in ConstraintLayout when executing Transition on
> DropDownList
> --------------------------------------------------------------------------------
>
> Key: FLEX-33311
> URL: https://issues.apache.org/jira/browse/FLEX-33311
> Project: Apache Flex
> Issue Type: Bug
> Components: Spark: DropDownList, Spark: Layout, Transitions
> Affects Versions: Apache Flex 4.8 (parity release)
> Reporter: Maxime Cowez
> Assignee: Mihai Chira
> Fix For: Apache Flex 4.16.0
>
> Attachments: Main.mxml, SpecifiableListWrapper.as,
> SpecifiableListWrapperHorizontalSkin.mxml, stack.xml
>
>
> In some very specific cases {{ConstraintLayout}} will throw a null pointer
> error when a component inside it is playing a transition. I tried a few
> things and found out that:
> * {{parseConstraints()}} creates a new {{rowBaselines}} Vector (or empties it
> if it exists)
> * some other process calls {{clearConstraintCache()}} in the middle of the
> execution of {{parseConstraints()}}, setting {{rowBaselines}} back to {{null}}
> * {{parseConstraints()}} calls {{parseElementConstraints()}} which tries to
> access elements in {{rowBaselines}} and throws the error
> I know very little about how Effects and Transitions work, so the "some other
> process" part is a bit of a mistery to me.
> *Steps to reproduce*:
> # Compile and run Main.mxml
> # Select the first item in the list
> # Click on the list and, before the resize transition finishes, click on it
> again.
> *Workaround*: subclass ConstraintLayout (or FormItemLayout), override
> `measure()`, put a try/cacth block around it and use this custom layout. Not
> exactly pretty but it works without apparent side effects.
> *Quick fix* (but probably not the ideal solution): just test whether
> {{rowBaselines}} exists before trying to access it in
> {{ConstraintLayout.parseElementConstraints()}}.
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