Currently, some Handles (ex: ExecAggStageHandle) use NonNull and others don't (ex: HostPortalHandle).

absl::Nonnull usage does not protect against crashes where functions are called on moved handles. For example, santiago.roche@mongodb.com gave this example:

Handle<Dummy> handle{ptr}; // ptr has been allocated
doFunctionTransferringOwnership(std::move(handle)); // we move the handle
handle.doFoo(); // what happens here? We'll segfault for sure. 

To avoid this, one option is to always call assertValid() when functions are called on a handle. If this is done, then usage of absl::Nonnull can probably be done away with. This should be discussed further.