This is a somewhat common crash affecting users in 4.17. Users have not provided any descriptions of their actions when the crash occurred.
Source Context
1660 {
1661 FScopedTransaction Transaction(LOCTEXT("MeshPaintMode_TexturePaint_Transaction", "Texture Paint"));
1662
1663 // For undo
1664 TextureData->PaintingTexture2D->SetFlags(RF_Transactional);
1665 TextureData->PaintingTexture2D->Modify();
1666
1667 // Store source art
1668 FColor* Colors = (FColor*)TextureData->PaintingTexture2D->Source.LockMip(0);
1669 check(TextureData->PaintingTexture2D->Source.CalcMipSize(0) == TexturePixels.Num() * sizeof(FColor));
1670 ***** FMemory::Memcpy(Colors, TexturePixels.GetData(), TexturePixels.Num() * sizeof(FColor));
1671 TextureData->PaintingTexture2D->Source.UnlockMip(0);
1672
1673 // If render target gamma used was 1.0 then disable SRGB for the static texture
1674 TextureData->PaintingTexture2D->SRGB = FMath::Abs(RenderTargetResource->GetDisplayGamma() - 1.0f) >= KINDA_SMALL_NUMBER;
1675
1676 TextureData->PaintingTexture2D->bHasBeenPaintedInEditor = true;
1677 }
repro steps currently unknown
Assertion failed: TextureData->PaintingTexture2D->Source.CalcMipSize(0) == TexturePixels.Num() * sizeof(FColor) [File:D:\Build\++UE4+Release-4.17+Compile\Sync\Engine\Source\Editor\MeshPaintMode\Private\PaintModePainter.cpp] [Line: 1670] UE4Editor_MeshPaintMode!FPaintModePainter::FinishPaintingTexture() [paintmodepainter.cpp:1671] UE4Editor_MeshPaint!IMeshPaintEdMode::InputKey() [imeshpaintmode.cpp:246] UE4Editor_UnrealEd!FEditorModeTools::InputKey() [editormodemanager.cpp:751] UE4Editor_UnrealEd!FEditorViewportClient::InputKey() [editorviewportclient.cpp:2280] UE4Editor_UnrealEd!FLevelEditorViewportClient::InputKey() [leveleditorviewport.cpp:2518] UE4Editor_Engine!FSceneViewport::OnMouseButtonUp() [sceneviewport.cpp:575] UE4Editor_Slate!SViewport::OnMouseButtonUp() [sviewport.cpp:200] UE4Editor_Slate!<lambda_1002768c627006711ef2f351a87ec0e7>::operator() [slateapplication.cpp:5199] UE4Editor_Slate!FEventRouter::Route<FReply,FEventRouter::FToLeafmostPolicy,FPointerEvent,<lambda_1002768c627006711ef2f351a87ec0e7> >() [slateapplication.cpp:232] UE4Editor_Slate!FSlateApplication::RoutePointerUpEvent() [slateapplication.cpp:5188] UE4Editor_Slate!FSlateApplication::ProcessMouseButtonUpEvent() [slateapplication.cpp:5690] UE4Editor_Slate!FSlateApplication::OnMouseUp() [slateapplication.cpp:5670] UE4Editor_Core!FWindowsApplication::ProcessDeferredMessage() [windowsapplication.cpp:1718] UE4Editor_Core!FWindowsApplication::DeferMessage() [windowsapplication.cpp:2161] UE4Editor_Core!FWindowsApplication::ProcessMessage() [windowsapplication.cpp:881] UE4Editor_Core!FWindowsApplication::AppWndProc() [windowsapplication.cpp:721] user32!UserCallWinProcCheckWow() user32!DispatchMessageWorker() UE4Editor_Core!FWindowsPlatformMisc::PumpMessages() [windowsplatformmisc.cpp:1009] UE4Editor!FEngineLoop::Tick() [launchengineloop.cpp:3151] UE4Editor!GuardedMain() [launch.cpp:166] UE4Editor!GuardedMainWrapper() [launchwindows.cpp:134] UE4Editor!WinMain() [launchwindows.cpp:210] UE4Editor!__scrt_common_main_seh() [exe_common.inl:253] kernel32!BaseThreadInitThunk() ntdll!RtlUserThreadStart()
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-49782 in the post.