SlateScriptingCommands plugin loads too late to receive MainFrame RegisterCommandList broadcast

This was reported on UDN.

TL;DR: A user has reported that a custom command cannot be added to the "MainFrame" context in the UUICommandsScriptingSubsystem from their plugin.

The problem seems to be that the SlateScriptingCommands subsystem is loaded too late. Because of this, it doesn't have a chance to start listening for broadcasts of registered command lists before some have already been broadcasted. This leads to the "MainFrame" system's broadcast never reaching the SlateScriptingCommands subsystem, causing this issue.

Note that the issue still happens if LoadingPhase is set to "EarliestPossible" in SlateScripting.uplugin.

• Enable the SlateScriptingCommands plugin by adding "SlateScripting" as a plugin dependency in FortniteGame.uproject.
• Put a breakpoint (A) at the FInputBindingManager::Get().RegisterCommandList call in FMainFrameModule::StartupModule().
• Put a breakpoint (B) in UUICommandsScriptingSubsystem::Initialize(FSubsystemCollectionBase& Collection).
• Start the game with an attached debugger.

Result: Breakpoint A is triggered before breakpoint B.
Expected: Breakpoint B is triggered before breakpoint A.

Since A is triggered before B, the SlateScriptingCommands subsystem has no chance to hook into MainFrame commands.