title: When I Generate signed APK, I got an issue
body: <p>I want to Generate signed APK, and I have compile 'com.squareup.okhttp3:okhttp:3.8.1'.<br>
  but It's happened some issue. It's looks like about "okhttp" issue.
  I don't know how to resolve it. I research many info, but its seems seldom info about this.</p>
  
  <p>Messages:</p>
  
  <pre><code>Information:Gradle tasks [:app:assembleFreeRelease]
  Warning:okhttp3.Address: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.Authenticator: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.Cache: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.Cache$2: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.Cache$CacheResponseBody: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.Cache$Entry: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.CacheControl: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.CertificatePinner: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.Challenge: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.Connection: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.ConnectionPool: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.ConnectionSpec: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.ConnectionSpec$Builder: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.Cookie: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.Dispatcher: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.FormBody: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.Handshake: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.Headers: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.HttpUrl: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.HttpUrl$Builder: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.Interceptor$Chain: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.MediaType: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.MultipartBody: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.MultipartBody$Builder: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.MultipartBody$Part: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.OkHttpClient: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.OkHttpClient$Builder: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.Request: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.Request$Builder: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.RequestBody: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.RequestBody$1: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.RequestBody$2: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.RequestBody$3: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.Response: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.Response$Builder: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.ResponseBody: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.ResponseBody$1: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.Route: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.WebSocket: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.internal.cache.CacheStrategy: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.internal.cache.DiskLruCache: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.internal.cache.DiskLruCache$Snapshot: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.internal.connection.RealConnection: can't find referenced class javax.annotation.Nullable
  Warning:okhttp3.package-info: can't find referenced class javax.annotation.ParametersAreNonnullByDefault
  Warning:okio.AsyncTimeout: can't find referenced class javax.annotation.Nullable
  Warning:okio.Buffer: can't find referenced class javax.annotation.Nullable
  Warning:okio.BufferedSource: can't find referenced class javax.annotation.Nullable
  Warning:okio.ByteString: can't find referenced class javax.annotation.Nullable
  Warning:okio.DeflaterSink: can't find referenced class org.codehaus.mojo.animal_sniffer.IgnoreJRERequirement
  Warning:okio.HashingSink: can't find referenced class javax.annotation.Nullable
  Warning:okio.Okio: can't find referenced class org.codehaus.mojo.animal_sniffer.IgnoreJRERequirement
  Warning:okio.Okio$4: can't find referenced class javax.annotation.Nullable
  Warning:okio.RealBufferedSource: can't find referenced class javax.annotation.Nullable
  Warning:okio.Segment: can't find referenced class javax.annotation.Nullable
  Warning:okio.SegmentPool: can't find referenced class javax.annotation.Nullable
  Warning:okio.package-info: can't find referenced class javax.annotation.ParametersAreNonnullByDefault
  Warning:there were 169 unresolved references to classes or interfaces.
  Warning:Exception while processing task java.io.IOException: Please correct the above warnings first.
  Error:Execution failed for task ':app:transformClassesAndResourcesWithProguardForFreeRelease'. Job failed, see logs for details
  Information:BUILD FAILED
  Information:Total time: 9.693 secs
  Information:1 error
  Information:58 warnings
  Information:See complete output in console
  </code></pre>
  
  <p>How can I resolve this issue??</p>