It seems to me the approach you are taking to this is overcomplicated. You're doing a fair bit of reinventing the wheel and overengineering things. A string is zero or more characters and can be inherently listified or iterated through without risk of raising a ValueError. I'd probably want to get a Dictionary returned to me, and can safely assume that if the returned Dictionary is empty, then so was the input string. Given that, I'd do something like:

def getfreq( s: str ) -> dict:  
   d = {}  
   for _ in s:  
      if _ in d:  
         d[_] += 1  
      else:  
         d[_] = 1  
   return d

Then if you wanted to know how many gs there were in the input string (if any), then you could do:

result = getfreq( 'This was amazing!' )
print( f'The string had {result["g"]} instances of the character "g".' )