Reproduction steps:
1. Download user's project and open in Unity
2. Try moving Unity editor's window

Expected: Editor window moves fluently
Actual: Editor window lags and errors appear in the console

Reproduced on: 2017.1.3p3; 2017.2.2p3; 2017.4.1f1; 2018.1.0a4
Not reproduced on: 2018.1.0a5; 2018.1.0f1

Note:
1. Errors in the console:
"NullReferenceException: Object reference not set to an instance of an object"
"GUI Error: You are pushing more GUIClips than you are popping. Make sure they are balanced)"
2. In 2018.2.0a1 - 2018.2.0b1 does not lag but shows error in console: "Unloading broken assembly Assets/ZiosThemes.dll, this assembly can cause crashes in the runtime"
3. Sometimes crashes with stack trace:
0x0000000141047B83 (Unity) GUIStyle::Draw
0x0000000141846E30 (Unity) GUIStyle_CUSTOM_Internal_Draw
0x000000003B90BA8D (Mono JIT Code) (wrapper managed-to-native) UnityEngine.GUIStyle:Internal_Draw (UnityEngine.GUIContent,UnityEngine.Internal_DrawArguments&)
0x000000003B90B98A (Mono JIT Code) [C:\buildslave\unity\build\Runtime\IMGUI\Managed\GUIStyle.cs:341] UnityEngine.GUIStyle:Internal_Draw (intptr,UnityEngine.Rect,UnityEngine.GUIContent,bool,bool,bool,bool)
0x000000003B90CA61 (Mono JIT Code) [C:\buildslave\unity\build\Runtime\IMGUI\Managed\GUIStyle.cs:354] UnityEngine.GUIStyle:Draw (UnityEngine.Rect,bool,bool,bool,bool)
0x000000003B91253F (Mono JIT Code) [C:\buildslave\unity\build\Editor\Mono\GUI\Toolbar.cs:319] UnityEditor.Toolbar:OnGUI ()
0x00000000138F6F22 (Mono JIT Code) (wrapper runtime-invoke) object:runtime_invoke_void__this__ (object,intptr,intptr,intptr)
0x00007FFDB668601F (mono) [c:\buildslave\mono\build\mono\mini\mini.c:4937] mono_jit_runtime_invoke
0x00007FFDB65D8791 (mono) [c:\buildslave\mono\build\mono\metadata\object.c:2623] mono_runtime_invoke
0x000000014141F285 (Unity) scripting_method_invoke
0x0000000141417541 (Unity) ScriptingInvocation::Invoke
0x000000014104E0B3 (Unity) MonoBehaviourDoGUI
0x0000000141040950 (Unity) IMGUIModule::MonoBehaviourDoGUI
0x000000014118888D (Unity) MonoBehaviour::DoGUI
0x0000000140029F66 (Unity) GUIView::OnInputEvent
0x00000001401AD313 (Unity) GUIView::ProcessInputEvent
0x0000000140006D06 (Unity) GUIView::DoPaint
0x00000001400135C0 (Unity) GUIView::RepaintAll
0x000000014012CECA (Unity) Application::TickTimer
0x0000000141AA508D (Unity) MainMessageLoop
0x0000000141AA699E (Unity) WinMain
0x0000000141D0BD4C (Unity) __tmainCRTStartup
0x00007FFDE2DB1FE4 (KERNEL32) BaseThreadInitThunk
0x00007FFDE523F061 (ntdll) RtlUserThreadStart