NoSuchMethodError: The method '[]' was called on null.

[brhomapps]

this summery of errors

When I send a request to create a chat and the other person agrees to it, an error appears in the message screen.
And when I searched for the cause of the error, I found that in the collection of rooms > members in Firebase, the receiver ID is not obtained! it's null, And when I add the sender ID manually, the problem is solved, so please help me how to get the receiver ID in the members automatically??

I/flutter (22322): NoSuchMethodError: The method '[]' was called on null.
I/flutter (22322): Receiver: null
I/flutter (22322): Tried calling:

look at the image

screen shut of firebase
(https://drive.google.com/file/d/1cUbGYR_VBYddoRR4Vc2UhGcI3UvLaZ78/view?usp=sharing)

screen shut of emulator
(https://drive.google.com/file/d/17do-Wilzw0Oh84mYzDJ0OLfi2H0i-D4_/view?usp=sharing)

screen shut of android studio
(https://drive.google.com/file/d/1_m1rvh1xYpG8V746694iDhtsSHvlxyE5/view?usp=sharing)