Set target api to 28.

dependencies.gradle

@@ -1,7 +1,7 @@
 ext.versions = [
         minSdk               : 14,
         targetSdk            : 22,
-        compileSdk           : 27,
+        compileSdk           : 28,
         gradle               : '4.5.1',
 
         androidTools         : "3.1.3",

subsonic-api-image-loader/src/main/kotlin/org/moire/ultrasonic/subsonic/loader/image/AvatarRequestHandler.kt

@@ -23,6 +23,7 @@ class AvatarRequestHandler(
 
     override fun load(request: Request, networkPolicy: Int): Result {
         val username = request.uri.getQueryParameter(QUERY_USERNAME)
+            ?: throw IllegalArgumentException("Nullable username")
 
         val response = apiClient.getAvatar(username)
         if (response.hasError()) {

subsonic-api-image-loader/src/main/kotlin/org/moire/ultrasonic/subsonic/loader/image/CoverArtRequestHandler.kt

@@ -20,7 +20,7 @@ class CoverArtRequestHandler(private val apiClient: SubsonicAPIClient) : Request
     }
 
     override fun load(request: Request, networkPolicy: Int): Result {
-        val id = request.uri.getQueryParameter(QUERY_ID)
+        val id = request.uri.getQueryParameter(QUERY_ID) ?: throw IllegalArgumentException("Nullable id")
 
         val response = apiClient.getCoverArt(id)
         if (response.hasError()) {