Reproduction steps:
1. Open the attached project "OnGUIBugReport_U20173.zip";
2. Open the "Test" scene;
3. Enter a play mode;
4. Try to resize the Game view.

Expected results: the result of internetReachability will be displayed and resizing the Game window won't affect it.
Actual results: the result of internetReachability is displayed, but after resizing the window, errors in the Console start appearing.

Reproduced on: 2017.2.0f3, 2017.2p2, 2017.3.0b9, 2018.1.0a4.
Not reproduced on: 2017.2.0f2, 2017.1.2p3 and lower.
Regression has started: 2017.2.0f3.

Note: Error, which is shown in the console:
GUI Window tried to begin rendering while something else had not finished rendering! Either you have a recursive OnGUI rendering, or the previous OnGUI did not clean up properly.
NullReferenceException: Object reference not set to an instance of an object UnityEngine.GUILayoutUtility.Layout () (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:147) UnityEngine.GUIUtility.EndGUI (Int32 layoutType) (at C:/buildslave/unity/build/Runtime/IMGUI/Managed/GUIUtility.cs:228)
Assertion failed: Assertion failed on expression: 'device.IsInsideFrame()'

Fixed in 2018.2