[Bug]: Add-ons with javascript assets can cause clients on the same machine connect to hang indefinitely or exception when loaded

[fishface60]

Describe the Bug

When connecting a client on the same machine I can reliably get MapTool to indefinitely wait for an asset to load.

When creating a new campaign without the add-on loaded, connecting the client, and then loading the add-on sometimes an exception is shown.

To Reproduce

1. Download assets.zip
2. Start MapTool
3. Import the properties, add the add-on and drag the token onto the map.
4. Start a server
5. Start MapTool again
6. Connect MapTool to the local server as a player
7. Observe the hang

Alternatively:

2. Start MapTool
3. Import the properties and drag the token onto the map
4. Start a server
5. Start MapTool again
6. Connect MapTool to the local server as a player
7. Add the add-on
8. Repeat the process until the following error is logged to console and a pop-up is shown.

19:42:36.391 (MapTool.java:298) [SocketConnection.ReceiveThread] ERROR - Error receiving game data update.
java.util.concurrent.ExecutionException: net.rptools.maptool.model.gamedata.InvalidDataOperation: Data Namespace addon: does not exist for type com.gitlab.richardmaw.mt-gurps.
        at java.base/java.util.concurrent.CompletableFuture.reportGet(CompletableFuture.java:396) ~[?:?]
        at java.base/java.util.concurrent.CompletableFuture.get(CompletableFuture.java:2073) ~[?:?]
        at net.rptools.maptool.model.gamedata.GameDataImporter.importData(GameDataImporter.java:97) ~[main/:?]
        at net.rptools.maptool.client.ClientMessageHandler.handle(ClientMessageHandler.java:449) [main/:?]
        at net.rptools.maptool.client.ClientMessageHandler.handleMessage(ClientMessageHandler.java:151) [main/:?]
        at net.rptools.clientserver.simple.connection.AbstractConnection.dispatchMessage(AbstractConnection.java:109) [clientserver.jar:?]
        at net.rptools.clientserver.simple.connection.AbstractConnection.dispatchCompressedMessage(AbstractConnection.java:115) [clientserver.jar:?]
        at net.rptools.clientserver.simple.connection.SocketConnection$ReceiveThread.run(SocketConnection.java:164) [clientserver.jar:?]
Caused by: net.rptools.maptool.model.gamedata.InvalidDataOperation: Data Namespace addon: does not exist for type com.gitlab.richardmaw.mt-gurps.
        at net.rptools.maptool.model.gamedata.InvalidDataOperation.createNamespaceDoesNotExist(InvalidDataOperation.java:110) ~[main/:?]
        at net.rptools.maptool.model.gamedata.MemoryDataStore.setData(MemoryDataStore.java:188) ~[main/:?]
        at net.rptools.maptool.model.gamedata.MemoryDataStore.lambda$setProperty$9(MemoryDataStore.java:212) ~[main/:?]
        at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1768) ~[?:?]
        at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1760) ~[?:?]
        at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:387) ~[?:?]
        at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1312) ~[?:?]
        at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1843) ~[?:?]
        at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1808) ~[?:?]
        at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:188) ~[?:?]
19:42:36.408 (I18N.java:181) [SocketConnection.ReceiveThread] DEBUG - Cannot find key 'Error receiving game data update.<br/>java.util.concurrent.ExecutionException: net.rptools.maptool.model.gamedata.InvalidDataOperation: Data Namespace addon: does not exist for type com.gitlab.richardmaw.mt-gurps.' in properties file.

Expected Behaviour

Running a second MapTool instance locally to test player client behaviour neither hangs nor displays error messages.

Screenshots

No response

MapTool Info

First observed in v1.14.3 but present in develop

Desktop

Ubuntu 24.04.1 LTS

Additional Context

Current hypothesis for the exception is that the asset raced the task for creating the add-on and the asset was loaded before the add-on was created.