Clean up code, took a lint ;)

app/proguard-rules.pro

@@ -21,9 +21,35 @@
 # hide the original source file name.
 -renamesourcefileattribute SourceFile
 
-# For SerializedName
+##---------------Begin: proguard configuration for Gson  ----------
+# Gson uses generic type information stored in a class file when working with fields. Proguard
+# removes such information by default, so configure it to keep all of it.
+-keepattributes Signature
+
+# For using GSON @Expose annotation
 -keepattributes *Annotation*
+
+# Gson specific classes
+-dontwarn sun.misc.**
+#-keep class com.google.gson.stream.** { *; }
+
+# Application classes that will be serialized/deserialized over Gson
+-keep class com.google.gson.examples.android.model.** { <fields>; }
+
+# Prevent proguard from stripping interface information from TypeAdapter, TypeAdapterFactory,
+# JsonSerializer, JsonDeserializer instances (so they can be used in @JsonAdapter)
+-keep class * extends com.google.gson.TypeAdapter
+-keep class * implements com.google.gson.TypeAdapterFactory
+-keep class * implements com.google.gson.JsonSerializer
+-keep class * implements com.google.gson.JsonDeserializer
+
+# Prevent R8 from leaving Data object members always null
 -keepclassmembers,allowobfuscation class * {
   @com.google.gson.annotations.SerializedName <fields>;
 }
--keep,allowobfuscation @interface com.google.gson.annotations.SerializedName
+
+# Retain generic signatures of TypeToken and its subclasses with R8 version 3.0 and higher.
+-keep,allowobfuscation,allowshrinking class com.google.gson.reflect.TypeToken
+-keep,allowobfuscation,allowshrinking class * extends com.google.gson.reflect.TypeToken
+
+##---------------End: proguard configuration for Gson  ----------