UMG button open level fails

Hi, I have a button in UMG which should load a new level, but when I click it, I get the following error:

!Id:2a9a3e2b5781facbe0155b7558800bdc

Unknown exception - code 00000001 (first/second chance not available)

World //Game//Maps//UEDPIE_0_Main.Main not cleaned up by garbage collection!
(Object is not currently rooted)

KERNELBASE + 37901 bytes
UE4Editor_Core + 3095292 bytes
UE4Editor_Core + 1661738 bytes
UE4Editor_UnrealEd + 1828904 bytes
UE4Editor_Engine + 9029087 bytes
UE4Editor_Engine + 8827382 bytes
UE4Editor_Engine + 9123021 bytes
UE4Editor_UnrealEd + 1812794 bytes
UE4Editor_UnrealEd + 5925286 bytes
UE4Editor!FEngineLoop::Tick() + 3294 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.4\engine\source\runtime\launch\private\launchengineloop.cpp:2098]
UE4Editor!GuardedMain() + 476 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.4\engine\source\runtime\launch\private\launch.cpp:133]
UE4Editor!GuardedMainWrapper() + 26 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.4\engine\source\runtime\launch\private\windows\launchwindows.cpp:125]
UE4Editor!WinMain() + 249 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.4\engine\source\runtime\launch\private\windows\launchwindows.cpp:201]
UE4Editor!__tmainCRTStartup() + 329 bytes [f:\dd\vctools\crt\crtw32\dllstuff\crtexe.c:618]

I assume it is because I open a level before I return from the event?

https://answers.unrealengine.com/questions/84838/bug-44-umg-loading-level-w-umg-on-viewport-causes.html