Fix NullReferenceException in Assets.Merge when Discord sends null assets

[HarukaYamamoto0]

Summary

This PR fixes a NullReferenceException thrown inside Assets.Merge(Assets other) when Discord sends presence updates where the assets payload is null or missing.

The bug does not prevent Rich Presence updates from being applied in the Discord client, but it does cause spurious error logs and can be confusing for users embedding the library in games or mods.

Problem

DiscordRpcClient.ProcessMessage calls RichPresence.Merge to keep CurrentPresence in sync with incoming presence updates from Discord:

if (presenceMessage.Presence == null)
{
    CurrentPresence = null;
}
else if (CurrentPresence == null)
{
    CurrentPresence = new RichPresence().Merge(presenceMessage.Presence);
}
else
{
    CurrentPresence.Merge(presenceMessage.Presence);
}

Inside RichPresence.Merge, assets are merged like this (simplified):

if (presence.HasAssets())
{
    if (!HasAssets())
        Assets = presence.Assets;
    else
        Assets.Merge(presence.Assets);
}
else
{
    Assets = null;
}

When Discord sends a presence update without assets, presence.Assets can be null. In that situation, Assets.Merge(presence.Assets) is invoked with other == null, but Assets.Merge does not guard against this and unconditionally dereferences other:

internal void Merge(Assets other)
{
    // uses other._smallimagetext, other._largeimagekey, etc...
}

This leads to a NullReferenceException similar to:

Unhandled Exception while processing event: System.NullReferenceException
Object reference not set to an instance of an object.
   at DiscordRPC.Assets.Merge(Assets other)
   at DiscordRPC.RichPresence.Merge(BaseRichPresence presence)
   at DiscordRPC.DiscordRpcClient.ProcessMessage(IMessage message)
   at DiscordRPC.DiscordRpcClient.<.ctor>b__103_0(Object sender, IMessage msg)
   at DiscordRPC.RPC.RpcConnection.EnqueueMessage(IMessage message)

In practice:

• The Rich Presence update succeeds and is visible in the Discord client.
• The background RPC handling thread logs this exception whenever assets are cleared or updated in certain ways.

This is especially visible in game/mod environments where logs are surfaced prominently.

Reproduction

One way to reproduce:

1. Set a Rich Presence with valid Assets (large/small images).
2. Later, send an updated presence that clears or removes assets (e.g. presence without any asset keys).
3. When Discord sends back the presence update, Assets.Merge may be called with other == null, triggering a NullReferenceException in the RPC event loop.

This can happen in real applications that dynamically toggle or clear presence images.

Fix

The fix is minimal and fully backwards compatible:

• Add a guard clause at the beginning of Assets.Merge(Assets other) to handle other == null.
• When other is null, the method now resets local asset fields and returns early, effectively treating it as “clear all assets”.

internal void Merge(Assets other)
{
    // Guard against Discord sending null assets in presence updates
    if (other == null)
    {
        _smallimagetext = null;
        _smallimageurl = null;
        _largeimagetext = null;
        _largeimageurl = null;

        _largeimagekey = null;
        _smallimagekey = null;

        LargeImageID = null;
        SmallImageID = null;

        IsLargeImageKeyExternal = false;
        IsSmallImageKeyExternal = false;

        return;
    }

    // rest...
}

This aligns with the idea that presence updates without assets should simply clear them instead of causing an exception.

Impact

• No API changes.
• Existing behavior when other != null is unchanged.
• Presence updates without assets no longer cause NullReferenceException to be logged.
• The fix is compatible with all existing target frameworks, including fx 4.5, which is important for engines like Unity, MonoGame, Godot, and game mods embedding the library.

Testing

• Built the library targeting fx 4.5.

• Integrated the patched DiscordRPC.dll into a game mod environment.

• Verified:

  • Rich Presence updates with and without assets work as expected.
  • Toggling/clearing images no longer produces NullReferenceException in the logs.
  • No regressions observed in normal Rich Presence behavior.