While Bulding the light maps, The scenes compleats the building and then crashes. cant see the final result. I am using Speed tree as well. Its a exeterior scene setup

For more info, pasting the error report Please Help

MachineId:CCB42BFA47A2AA119D14B4A1BFA5ED97
EpicAccountId:9722bcdfcca54ea68af4d5e51d4a6182

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

Assertion failed: NewPosition >= 0 [File:D:\BuildFarm\buildmachine_++depot+UE4-Releases+4.7\Engine\Source\Runtime\Core\Private\Windows\WindowsPlatformFile.cpp] [Line: 59]

KERNELBASE + 26840 bytes
UE4Editor_Core!FOutputDeviceWindowsError::Serialize() + 292 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\runtime\core\private\windows\windowsplatformoutputdevices.cpp:95]
UE4Editor_Core!FOutputDevice::Logf__VA() + 248 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\runtime\core\private\misc\outputdevice.cpp:144]
UE4Editor_Core!FDebug::AssertFailed() + 1079 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\runtime\core\private\misc\outputdevice.cpp:224]
UE4Editor_Core!FFileHandleWindows::Seek() + 132 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\runtime\core\private\windows\windowsplatformfile.cpp:60]
UE4Editor_Core!FArchiveFileWriterGeneric::Seek() + 36 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\runtime\core\private\hal\filemanagergeneric.cpp:715]
UE4Editor_Core!FArchive::SerializeCompressed() + 4041 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\runtime\core\private\serialization\archive.cpp:690]
UE4Editor_CoreUObject!FUntypedBulkData::SerializeBulkData() + 172 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\runtime\coreuobject\private\serialization\bulkdata.cpp:1009]
UE4Editor_CoreUObject!UPackage::SavePackage() + 27727 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\runtime\coreuobject\private\uobject\savepackage.cpp:3455]
UE4Editor_UnrealEd!UEditorEngine::SavePackage() + 1105 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\editor\unrealed\private\editor.cpp:4416]
UE4Editor_UnrealEd!UEditorEngine::Exec_Obj() + 1739 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\editor\unrealed\private\editorserver.cpp:4162]
UE4Editor_UnrealEd!UEditorEngine::Exec() + 1160 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\editor\unrealed\private\editorserver.cpp:5238]
UE4Editor_UnrealEd!UUnrealEdEngine::Exec() + 273 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\editor\unrealed\private\unrealedsrv.cpp:742]
UE4Editor_UnrealEd!SaveWorld() + 4227 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\editor\unrealed\private\filehelpers.cpp:526]
UE4Editor_UnrealEd!FEditorFileUtils::AutosaveMap() + 590 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\editor\unrealed\private\filehelpers.cpp:2059]
UE4Editor_UnrealEd!FPackageAutoSaver::AttemptAutoSave() + 317 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\editor\unrealed\private\packageautosaver.cpp:178]
UE4Editor_UnrealEd!UUnrealEdEngine::Tick() + 145 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\editor\unrealed\private\unrealedengine.cpp:375]
UE4Editor!FEngineLoop::Tick() + 4179 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\runtime\launch\private\launchengineloop.cpp:2257]
UE4Editor!GuardedMain() + 1404 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\runtime\launch\private\launch.cpp:142]
UE4Editor!GuardedMainWrapper() + 26 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\runtime\launch\private\windows\launchwindows.cpp:126]
UE4Editor!WinMain() + 249 bytes [d:\buildfarm\buildmachine_++depot+ue4-releases+4.7\engine\source\runtime\launch\private\windows\launchwindows.cpp:202]
UE4Editor!__tmainCRTStartup() + 329 bytes [f:\dd\vctools\crt\crtw32\dllstuff\crtexe.c:618]