when you first start Unity all was well after the second run of Unity is such a mistake!
(I do not speak English - write simpler)
TypeLoadException: Could not load type 'PhotonNetwork' from assembly 'Assembly-CSharp, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'.
UnityEditor.InspectorWindow.DrawEditors (Boolean isRepaintEvent, UnityEditor.Editor[] editors, Boolean eyeDropperDirty) (at C:/BuildAgent/work/300357e52574df36/Editor/Mono/Inspector/InspectorWindow.cs:888)
UnityEditor.InspectorWindow.OnGUI () (at C:/BuildAgent/work/300357e52574df36/Editor/Mono/Inspector/InspectorWindow.cs:243)
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture)