Can't get Auto-ID Document in Firestore - firebase

Excuse me,
I'm working on my code about making a Document Firestore that could be delete, this is how my Firestore Database looks like :
Firestore Data Structure
I'd like to delete the Auto-ID Document after the Collection "X-MIPA.6". So, I followed the previous solution about "Firebase Cloud Firestore get auto-id of newly created document", so here is my code looks like :
My Code
But, when I tested, my app force closed and show me this Error :
The Error
How is it possible? How can I fix it?
I want to delete the Document based on the Auto-ID Document on my Firestore Database.
Thanks!

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You will need to attach document names to your documents before uploading them.
I think you are experiencing this issue because your documents are being given Auto Ids.
Try using the current timestamp as the document name. this will help arrange documents in order according to time uploaded.
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it gets created to
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