Try a stub since nullptr is being weird for some reason

src/core/hle/service/am/service/library_applet_accessor.cpp

@@ -28,7 +28,7 @@ ILibraryAppletAccessor::ILibraryAppletAccessor(Core::System& system_,
         {30, D<&ILibraryAppletAccessor::GetResult>, "GetResult"},
         {50, nullptr, "SetOutOfFocusApplicationSuspendingEnabled"},
         {60, D<&ILibraryAppletAccessor::PresetLibraryAppletGpuTimeSliceZero>, "PresetLibraryAppletGpuTimeSliceZero"},
-        {90, nullptr, "Unknown90"},
+        {90, D<&ILibraryAppletAccessor::Unknown90>, "Unknown90"},
         {100, D<&ILibraryAppletAccessor::PushInData>, "PushInData"},
         {101, D<&ILibraryAppletAccessor::PopOutData>, "PopOutData"},
         {102, nullptr, "PushExtraStorage"},
@@ -97,6 +97,11 @@ Result ILibraryAppletAccessor::Terminate() {
     R_SUCCEED();
 }
 
+Result ILibraryAppletAccessor::Unknown90() {
+    LOG_WARNING(Service_AM, "(STUBBED) called");
+    R_SUCCEED();
+}
+
 Result ILibraryAppletAccessor::PushInData(SharedPointer<IStorage> storage) {
     LOG_DEBUG(Service_AM, "called");
     m_broker->GetInData().Push(storage);

src/core/hle/service/am/service/library_applet_accessor.h

@@ -26,6 +26,7 @@ private:
     Result Start();
     Result RequestExit();
     Result Terminate();
+    Result Unknown90();
     Result PushInData(SharedPointer<IStorage> storage);
     Result PopOutData(Out<SharedPointer<IStorage>> out_storage);
     Result PushInteractiveInData(SharedPointer<IStorage> storage);