Steps to reproduce:

1. Open attached project "_IMGUIrepro.zip"
2. Open "M" scene
3. In search bar which is located in project tab input some text, for exmaple "TestException"
4. Run scene
5. Notice Uncaught exception:
Uncaught exception in async net callback: Object reference not set to an instance of an object
UnityEditor.AsyncHTTPClient:Done(State, Int32)
at UnityEngine.GUISkin.GetStyle (System.String styleName) [0x00010] in C:\buildslave\unity\build\Runtime\IMGUI\Managed\GUISkin.cs:319
at UnityEngine.GUIStyle.op_Implicit (System.String str) [0x00020] in C:\buildslave\unity\build\Runtime\IMGUI\Managed\GUIStyle.cs:586
at UnityEditor.ProjectBrowser.InitSearchMenu () [0x00014] in C:\buildslave\unity\build\Editor\Mono\ProjectBrowser.cs:483
at UnityEditor.ProjectBrowser.AssetStoreSearchEndedCallback () [0x00000] in C:\buildslave\unity\build\Editor\Mono\ProjectBrowser.cs:511
at UnityEditor.ObjectListArea+<QueryAssetStore>c__AnonStorey3E.<>m__60 (UnityEditor.AssetStoreSearchResults results) [0x00356] in C:\buildslave\unity\build\Editor\Mono\ObjectListArea.cs:406
at UnityEditor.AssetStoreResultBase`1[Derived].Parse (UnityEditor.AssetStoreResponse response) [0x000fc] in C:\buildslave\unity\build\Editor\Mono\AssetStore\AssetStoreClient.cs:89
at UnityEditor.AssetStoreClient+<SearchAssets>c__AnonStorey66.<>m__BF (UnityEditor.AssetStoreResponse ar) [0x00000] in C:\buildslave\unity\build\Editor\Mono\AssetStore\AssetStoreClient.cs:751
at UnityEditor.AssetStoreClient+<WrapJsonCallback>c__AnonStorey65.<>m__BD (UnityEditor.AsyncHTTPClient job) [0x00012] in C:\buildslave\unity\build\Editor\Mono\AssetStore\AssetStoreClient.cs:624
UnityEditor.AsyncHTTPClient:Done(State, Int32)

Workaround: make sure that search bar has no input when entering play mode

Note: this is only reproducible when any active script component has calling OnGUI(){}

Reproduced with: 5.1.5f1, 5.2.4f1, 5.3.4f1, 5.4.0p1, 5.5.0a5, 2018.1.0b1