在Kotlin上，杰克逊ObjectMapper.readValue（）导致java.lang.IllegalStateException：readValue（content，jacksonTypeRef <T> （））不能为null

Question

On crashlytics I am getting an java.lang.IllegalStateException: readValue(content, jacksonTypeRef<T>()) must not be null exception on the 4th line. I couldn't reproduce it. I think mapper.readValue(string) is returning null where is shouldn't. I handled the string being null case so other than string being null what could cause mapper.readValue(string) to return null so I end up with this exception and what is possible way to reproduce this error?

var token: token? = null
    get() {
       val string = sharedPreferences.getString(TOKEN_KEY, null)
       field = if (string == null) null else mapper.readValue(string)
       return field     
    }
    set(value) {
        field = value
        val json = mapper.writeValueAsString(value)
        sharedPreferences.edit().putString(TOKEN_KEY, json).apply()
    }

Answer 1

事实证明sharedPreferences.getString(TOKEN_KEY, null)在这种情况下， sharedPreferences.getString(TOKEN_KEY, null)返回“ null”字符串，而mapper.readValue(string)返回null 。