BUG #15841: Creating new database and collate error

The following bug has been logged on the website:

Bug reference:      15841
Logged by:          Kamil Giza
Email address:      kgizmo@wp.pl
PostgreSQL version: 12beta1
Operating system:   Windows 10
Description:

Hello,
While trying to create new database with encoding UTF8 and Colletion
Polish_Poland.1250 and Character type Polish_Poland.1250 I got an error: 

"ERROR: new encoding (UTF8) is incompatible with the encoding of the
template database (WIN1250)
HINT: Use the same encoding as in the template database, or use template0 as
template."

After choosing template0 database has been created (in version 11 I didn't
have to choose template) but when I try to get into that database using
pgAdmin 4 I got "INTERNAL SERVER ERROR : string indices must be integers".
The same error I got when try to expand "postgres" database.