Wait for complete images upload: Firebase Storage + Database

Hello!
In one screen the user can select several photos to upload via Firebase Storage Extension.
Then, the respective URLs will be sent to the Firebase Database.
The problem is: if the upload process is not immediately completed, the value in the Data Base is being sent empty. The question is: how can I configure for the “cal. Firebase_Database1. Store Value” to wait for complete List of all the URLs of the images selected for upload.
Help me, please!

show us what you have tried till now?