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I'm attempting to retrieve the id of a user who created a particular KnowledgeArticleVersion and getting an "attempt to de-reference a null object" error on the line where I add creator to modifiers. Using system debug to print out kavs[0].CreatedById, though, I get

15:58:27:020 USER_DEBUG [18]|DEBUG|005i0000000jqkeAAA

which is the ID I want and implies that what I'm attempting to add to this list is not null (In this case kavs was of length 1). I even threw in a check for nullity which didn't seem to make a difference.

List<KnowledgeArticleVersion> kavs = [SELECT CreatedById FROM KnowledgeArticleVersion WHERE (KnowledgeArticleId = :kas.Id AND PublishStatus = 'online' AND Language = 'en_US')];

List<String> modifiers;
for (Integer i = 0; i < kavs.size(); i++){
        if (kavs[i].CreatedById != null){
            creator = kavs[i].CreatedById;
            System.debug(creator);
            modifiers.add(creator);
        }
    }
String modifiedBy = String.join(modifiers, ' ,');

What am I missing? I've tried a ton of different changes with no success.

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You have declared List<String> modifiers;

But before instanciating as modifiers= new List<String>(); you are trying to add modifiers.add(creator);

So, modifiers is null and you are getting

Attempt to de-reference a null object

Better, declare List<String> modifiers = new List<String>(); to solve the issue.

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