NRE on core reboot if memory callback is set via Lua with mGBA

[CasualPokePlayer]

Other cores don't appear to have any issue.

Basic script to reproduce the issue

event.on_bus_write(function() end, 0)

System.NullReferenceException: Object reference not set to an instance of an object.
   at BizHawk.Emulation.Cores.Nintendo.GBA.MGBAMemoryCallbackSystem.Remove(CallbackContainer cb) in /src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBA/MGBAMemoryCallbackSystem.cs:line 113
   at BizHawk.Emulation.Cores.Nintendo.GBA.MGBAMemoryCallbackSystem.Remove(MemoryCallbackDelegate action) in /src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBA/MGBAMemoryCallbackSystem.cs:line 144
   at BizHawk.Client.Common.LuaFunctionList.RemoveInner(NamedLuaFunction function) in /src/BizHawk.Client.Common/lua/LuaFunctionList.cs:line 40
   at BizHawk.Client.Common.LuaFunctionList.RemoveForFile(LuaFile file) in /src/BizHawk.Client.Common/lua/LuaFunctionList.cs:line 47
   at BizHawk.Client.EmuHawk.LuaConsole.DisableLuaScript(LuaFile file) in /src/BizHawk.Client.EmuHawk/tools/Lua/LuaConsole.cs:line 1562
   at BizHawk.Client.EmuHawk.LuaConsole.Restart() in /src/BizHawk.Client.EmuHawk/tools/Lua/LuaConsole.cs:line 221
   at BizHawk.Client.EmuHawk.ToolManager.Restart(Config config, IEmulator emulator, IGameInfo game) in /src/BizHawk.Client.EmuHawk/tools/ToolManager.cs:line 607
   at BizHawk.Client.EmuHawk.MainForm.LoadRomInternal(String path, LoadRomArgs args, Boolean& failureIsFromAskSave) in /src/BizHawk.Client.EmuHawk/MainForm.cs:line 3804
   at BizHawk.Client.EmuHawk.MainForm.LoadRom(String path, LoadRomArgs args, Boolean& failureIsFromAskSave) in /src/BizHawk.Client.EmuHawk/MainForm.cs:line 3575
   at BizHawk.Client.EmuHawk.MainForm.RebootCore() in /src/BizHawk.Client.EmuHawk/MainForm.cs:line 1338
   at System.Windows.Forms.ToolStripItem.RaiseEvent(Object key, EventArgs e)
   at System.Windows.Forms.ToolStripMenuItem.OnClick(EventArgs e)
   at System.Windows.Forms.ToolStripItem.HandleClick(EventArgs e)
   at System.Windows.Forms.ToolStripItem.HandleMouseUp(MouseEventArgs e)
   at System.Windows.Forms.ToolStrip.OnMouseUp(MouseEventArgs mea)
   at System.Windows.Forms.ToolStripDropDown.OnMouseUp(MouseEventArgs mea)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ToolStrip.WndProc(Message& m)
   at System.Windows.Forms.ToolStripDropDown.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

[SuuperW]

Probably fixed with #4630.

[Morilli]

This reminds me of #4157, and could be fixed by clearing all memory callbacks in MGBAMemoryCallbackSystem::Dispose or alternatively checking whether the core is disposed (or null I guess) in Remove.

I don't know whether it's even valid to have such a disposed object be "active" from the context of the lua console though but I guess that's what #4629 is about.