Use the 'Remove explicit type arguments' quick-fix to clean up the code.
Examples:
// 'String' type can be inferred here
fun foo(): MutableList<String> = mutableListOf<String>()
// Here 'String' cannot be inferred, type argument is required.
fun bar() = mutableListOf<String>()
After the quick-fix is applied:
fun foo(): MutableList<String> = mutableListOf() <== Updated
fun bar() = mutableListOf<String>()