Wait for complete images upload: Firebase Storage + Database

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?