Finding the most common character in a string written in Go

Few days ago I started experiments with GO language and I decided to resolve one problem with it.
There is my first function on GО which search for most common character in a given string.
The goal was to work with cyrillic as well as latin for that. Тhe code below will explain the rest.
func getTheMostCommonChar(text string) (string, error) {

	var textLen int = utf8.RuneCountInString(text)

	if textLen == 0 {
		return "",  errors.New("Empty string given")
	}

	var i int = 0
	store := make(map[string]int)

	// collect char as key and value times it exists in the text
	for i < textLen{
		store[string([]rune(text)[i])]++
		i++
	}

	var mostCommon string

	for key, value := range store {
		if (value > store[mostCommon]) {
			mostCommon = key
		}
	}

	return mostCommon, nil
}
You can test it in Go playground