Before asking the question, I searched for a similar question on StackOverflow:
file was loaded in the wrong encoding: 'UTF-8' in android studio
But
in my case , when i uploaded the first .ttf file in assert folder , android studio asked me to choose one file type , and when I selected text type . this problem start to appear . the right file type is : 'Files Opened In Associated Applications'. to fix your problem , you have to open preferences ->Editor ->File Types and search for .ttf extension then remove it and add it again under 'Files Opened In Associated Applications'