Fix build warnings

Fix build warnings introduced after the update to FlaUI 5, which adds
nullable support.

Author: aristotelos

src/FlaUI.WebDriver/Controllers/ElementController.cs

@@ -174,11 +174,11 @@ public async Task<ActionResult> IsElementSelected([FromRoute] string sessionId,
             var isSelected = false;
             if (element.Patterns.SelectionItem.IsSupported)
             {
-                isSelected = element.Patterns.SelectionItem.PatternOrDefault.IsSelected.ValueOrDefault;
+                isSelected = element.Patterns.SelectionItem.TryGetPattern(out var pattern) && pattern.IsSelected.ValueOrDefault;
             }
             else if (element.Patterns.Toggle.IsSupported)
             {
-                isSelected = element.Patterns.Toggle.PatternOrDefault.ToggleState.ValueOrDefault == Core.Definitions.ToggleState.On;
+                isSelected = element.Patterns.Toggle.PatternOrDefault?.ToggleState.ValueOrDefault == Core.Definitions.ToggleState.On;
             }
             return await Task.FromResult(WebDriverResult.Success(isSelected));
         }

src/FlaUI.WebDriver/Controllers/SessionController.cs

@@ -91,7 +91,7 @@ public async Task<ActionResult> CreateNewSession([FromBody] CreateSessionRequest
                     if (appPath.EndsWith("!App"))
                     {
                         isAppOwnedBySession = true;
-                        return Application.LaunchStoreApp(appPath, appArguments);
+                        return Application.LaunchStoreApp(appPath, appArguments ?? "");
                     }
 
                     var processStartInfo = new ProcessStartInfo(appPath, appArguments ?? "");