Fix for the detection of the currently active non-EVE app

Eve-O-Preview/Services/Implementation/ThumbnailManager.cs

@@ -195,9 +195,14 @@ namespace EveOPreview.Services
 				foregroundWindowTitle = foregroundView.Title;
 			}
 			else if (!isClientWindow)
+			{
+				// Under some circumstances Foreground WindowHandle can be zero
+				// (f.e. when Thumbnail is silently stealing focus from the currently open app)
+				if (foregroundWindowHandle != IntPtr.Zero)
 				{
 					this._externalApplication = foregroundWindowHandle;
 				}
+			}
 
 			// No need to minimize EVE clients when switching out to non-EVE window (like thumbnail)
 			if (!string.IsNullOrEmpty(foregroundWindowTitle))