HLE/APT: Always return an error from PrepareToStartNewestHomeMenu so that the Home Menu doesn't try to reboot the system.

As per 3dbrew:
"During Home Menu start-up it uses APT:PrepareToStartNewestHomeMenu. If that doesn't return an error(normally NS returns 0xC8A0CFFC for that), Home Menu starts a hardware reboot with APT:StartNewestHomeMenu etc. "

src/core/hle/service/apt/apt.cpp

@@ -776,6 +776,20 @@ void PrepareToStartLibraryApplet(Service::Interface* self) {
     LOG_DEBUG(Service_APT, "called applet_id=%08X", applet_id);
 }
 
+void PrepareToStartNewestHomeMenu(Service::Interface* self) {
+    IPC::RequestParser rp(Kernel::GetCommandBuffer(), 0x1A, 0, 0); // 0x1A0000
+    IPC::RequestBuilder rb = rp.MakeBuilder(1, 0);
+
+    // TODO(Subv): This command can only be called by a System Applet (return 0xC8A0CC04 otherwise).
+
+    // This command must return an error when called, otherwise the Home Menu will try to reboot the
+    // system.
+    rb.Push(ResultCode(ErrorDescription::AlreadyExists, ErrorModule::Applet,
+                       ErrorSummary::InvalidState, ErrorLevel::Status));
+
+    LOG_DEBUG(Service_APT, "called");
+}
+
 void PreloadLibraryApplet(Service::Interface* self) {
     IPC::RequestParser rp(Kernel::GetCommandBuffer(), 0x16, 1, 0); // 0x160040
     AppletId applet_id = static_cast<AppletId>(rp.Pop<u32>());

src/core/hle/service/apt/apt.h

@@ -419,6 +419,16 @@ void GetAppCpuTimeLimit(Service::Interface* self);
  */
 void PrepareToStartLibraryApplet(Service::Interface* self);
 
+/**
+ * APT::PrepareToStartNewestHomeMenu service function
+ *  Inputs:
+ *      0 : Command header [0x001A0000]
+ *  Outputs:
+ *      0 : Return header
+ *      1 : Result of function
+ */
+void PrepareToStartNewestHomeMenu(Service::Interface* self);
+
 /**
  * APT::PreloadLibraryApplet service function
  *  Inputs:

src/core/hle/service/apt/apt_s.cpp

@@ -17,7 +17,7 @@ const Interface::FunctionInfo FunctionTable[] = {
     {0x00060040, GetAppletInfo, "GetAppletInfo"},
     {0x00070000, nullptr, "GetLastSignaledAppletId"},
     {0x00080000, nullptr, "CountRegisteredApplet"},
-    {0x00090040, nullptr, "IsRegistered"},
+    {0x00090040, IsRegistered, "IsRegistered"},
     {0x000A0040, nullptr, "GetAttribute"},
     {0x000B0040, InquireNotification, "InquireNotification"},
     {0x000C0104, nullptr, "SendParameter"},
@@ -34,7 +34,7 @@ const Interface::FunctionInfo FunctionTable[] = {
     {0x00170040, nullptr, "FinishPreloadingLibraryApplet"},
     {0x00180040, PrepareToStartLibraryApplet, "PrepareToStartLibraryApplet"},
     {0x00190040, nullptr, "PrepareToStartSystemApplet"},
-    {0x001A0000, nullptr, "PrepareToStartNewestHomeMenu"},
+    {0x001A0000, PrepareToStartNewestHomeMenu, "PrepareToStartNewestHomeMenu"},
     {0x001B00C4, nullptr, "StartApplication"},
     {0x001C0000, nullptr, "WakeupApplication"},
     {0x001D0000, nullptr, "CancelApplication"},