The generated code performs an explicit null check before calling

substring

, so you'd get a different exception in that case:

@NotNull
   public static final String getDesc(@NotNull Class $receiver) {
      Intrinsics.checkParameterIsNotNull($receiver, "$receiver");
      if (Intrinsics.areEqual($receiver, Void.TYPE)) {
         return "V";
      } else {
         String var10000 = createArrayType($receiver).getName();
         Intrinsics.checkExpressionValueIsNotNull(var10000, "createArrayType().name");
         String var1 = var10000;
         byte var2 = 1;
         if (var1 == null) {
            throw new TypeCastException("null cannot be cast to non-null type java.lang.String");
         } else {
            var10000 = var1.substring(var2);
            Intrinsics.checkExpressionValueIsNotNull(var10000, "(this as java.lang.String).substring(startIndex)");
            return StringsKt.replace$default(var10000, '.', '/', false, 4, (Object)null);
         }
      }
   }

And that bytecode was generated with 1.3.11 or with 1.2.71?

1.2.71 I believe

then I'm out of ideas 😄

Can you try to step trough it with a debugger?

Unfortunately the problem has only been observed so far in a production setting - I can't reproduce it in a test case.