How to reproduce:
1. Open the attached project
2. Set Game window resolution to "Standalone (1024x768)"
3. Enter Play mode
4. Click the 'Crash' button

Expected result: Editor does not crash, there is a warning that ScreenCapture.CaptureScreenshotAsTexture should only be called once the frame has ended.
Actual result: Editor crashes.

Reproducible with - 2017.3.0b1, 2017.4.6f1, 2018.1.7f1, 2018.2.0b11, 2018.3.0a3

Notes:
- Could not be tested on 2017.1 and 2017.2, since the above-mentioned method was introduced on 2017.3.
- The issue was not reproducible with OpenGL, or when rendering using integrated Intel graphics card.

Stack trace:
(Unity) ScreenCapture::CaptureScreenshotAsTexture
(Unity) ScreenCapture_CUSTOM_CaptureScreenshotAsTexture
(Mono JIT Code) (wrapper managed-to-native) UnityEngine.ScreenCapture:CaptureScreenshotAsTexture (int)
(Mono JIT Code) [C:\Users\UnityQA\Desktop\1053476_\Assets\SomeImgui.cs:9] SomeImgui:OnGUI ()
(Mono JIT Code) (wrapper runtime-invoke) object:runtime_invoke_void__this__ (object,intptr,intptr,intptr)
(mono) [c:\buildslave\mono\build\mono\mini\mini.c:4937] mono_jit_runtime_invoke
(mono) [c:\buildslave\mono\build\mono\metadata\object.c:2623] mono_runtime_invoke
(Unity) scripting_method_invoke
(Unity) ScriptingInvocation::Invoke
(Unity) MonoBehaviourDoGUI
(Unity) IMGUIModule::MonoBehaviourDoGUI
<...>