Did you follow this guide ?
https://appinventor.mit.edu/explore/ai2/clouddb-chat
If not, perhaps you should, then once you have the basics working, you can then start to add features.
You really do not need to use two screens to do this.
There is no need to encode to base64, just store the plain text
Here is an alternative way to provide an icon (image for the users) that might be helpful for you chat program development Chat app message style - #20 by SteveJG .
Here are links to our community discussions concerning building an App Inventor chat app . Some of these discussions might contain information that can help you to decide how you want to build YOUR chat
I made a block that decode selected elements in list view of base 64
When I lunch the app the app is telling
"Bad base64"
Looks like the Listview1.Selection is not a base64 string.
You may want to check this.
That is bas64 string
I want to make a button function that
If the button clicked one time before then open other window
Please help me 
Before you get completely tied up with multiple screens, you might want to consider using virtual screens - using vertical arrangements and changing their visibility.
Here is an example, using tabs:
https://groups.google.com/g/mitappinventortest/c/K-zAr2X3nio/m/qzrDox_pBgAJ
and a tutorial
Do you mean by having second click? You need to maintain click count.
if clickCount == 2 switch Screen and when you come back this screen back, reassign to 0.
If button clicked one time before then don't open screen 1 directly open screen 2
Please share your work, what you have done and also specify your requirement with more clarity.
I am making a login system
In This login system I am taking name and with a start value it open another screen and store it in tiny db when I close the app and reopen it need name again for this reason I want to make that if button clicked one time it directly open screen 2
Once button clicked save the required details in tinydb
So next time while screen initialise call this tag value and make changes accordingly
Show what you have made so far.
Welcome. Is it possible to give me this extension?
Which extension ?
base64
Thanks
