Relevant stack trace:

[docker] Caused by: java.lang.NullPointerException
[docker] at com.android.tools.r8.s.d.a(:15)
[docker] at com.android.tools.r8.s.d.a(:21)
[docker] at com.android.tools.r8.s.d.a(:9)
[docker] at com.android.tools.r8.s.d.a(:77)
[docker] at com.android.tools.r8.s.d.a(:178)
[docker] at com.android.tools.r8.R8.a(:223)
[docker] at com.android.tools.r8.R8.c(:1)
[docker] at com.android.tools.r8.R8.b(:1)
[docker] at com.android.tools.r8.utils.t.a(:23)
[docker] at com.android.tools.r8.R8.runForTesting(:3)
[docker] at com.android.tools.r8.R8.run(:3)
[docker] at com.android.builder.dexing.R8Tool.runR8(r8Tool.kt:199)

Reported as Gradle 6.1.1, build tools 29.0.2 and AGP plugin 3.5.3.
AGP-3.5.3 ships with R8 1.5.69

Retraced stacktrace for 1.5.69:

Caused by: java.lang.NullPointerException
at com.android.tools.r8.optimize.MemberRebindingAnalysis.firstLibraryClassForInterfaceTarget(MemberRebindingAnalysis.java:80)
at com.android.tools.r8.optimize.MemberRebindingAnalysis.validTargetFor(MemberRebindingAnalysis.java:54)
at com.android.tools.r8.optimize.MemberRebindingAnalysis.computeMethodRebinding(MemberRebindingAnalysis.java:164)
at com.android.tools.r8.optimize.MemberRebindingAnalysis.run(MemberRebindingAnalysis.java:341)
at com.android.tools.r8.R8.run(R8.java:425)
at com.android.tools.r8.R8.run(R8.java:260)
at com.android.tools.r8.R8.lambda$runForTesting$1(R8.java:251)
at com.android.tools.r8.utils.ExceptionUtils.withCompilationHandler(ExceptionUtils.java:61)
at com.android.tools.r8.utils.ExceptionUtils.withR8CompilationHandler(ExceptionUtils.java:49)
at com.android.tools.r8.R8.runForTesting(R8.java:247)
at com.android.tools.r8.R8.run(R8.java:152)

NPE location: https://r8.googlesource.com/r8/+/refs/tags/1.5.69/src/main/java/com/android/tools/r8/optimize/MemberRebindingAnalysis.java#80