he ‘kotlin-android-extensions‘ Gradle plugin is deprecated. Please use this migration guide

将 

plugins {
    id 'com.android.application'
    id 'kotlin-android'
    id 'kotlin-android-extensions'
    id 'com.mob.sdk'
    id 'fastdex.app'
}

  移除

  id 'kotlin-android-extensions' 
 

替换为

android {
    ...
    buildFeatures {
        viewBinding true
    }
}

替换findviewById

官方文档：https://developer.android.com/topic/libraries/view-binding/migration