Refactor RTMediaAdmin.php to handle JSON decoding error

[the-hercules]

#2054

Types of changes

What types of changes do your code introduce to the project?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update

Details

• How this issue fixed/resolved
  • In RTMediaAlbum.php , it was running json_decode 2 times on same string causes TypeError. Removing on json_decode fixes this issue.

Closing issues

Fixes: #2054