proguard problem after updating to android gradle plugin 3.5

I also got the same error.

And I found that it's an error of enabledR8 which is used to reduce app size.

and It is true predefined. So by set value, a false error was gone.

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Set it value false of r8 in gradle.properties file.

android.enableR8=false

I should exclude cached objects from obfuscating but temporarily I disabled R8 and enabled proguard, by adding this line to gradle.properties file:

android.enableR8=false

And adding this line to app build.gradle file:

useProguard true
minifyEnabled true

And the problem solved.