Protect more places against not having a XamlRoot (#19753)

We're seeing some crashes where we call `GetFocusedElement(null)`.

MSFT-54686865
MSFT-53824167
Possibly also #19751

(cherry picked from commit 0ee3cfb)
Service-Card-Id: PVTI_lADOAF3p4s4AxadtzgjzNUI
Service-Version: 1.23

Author: DHowett

src/cascadia/TerminalApp/TerminalWindow.cpp

@@ -888,6 +888,11 @@ namespace winrt::TerminalApp::implementation
         {
             // Manually bubble the OnDirectKeyEvent event up through the focus tree.
             auto xamlRoot{ _root->XamlRoot() };
+            if (!xamlRoot)
+            {
+                return false;
+            }
+
             auto focusedObject{ Windows::UI::Xaml::Input::FocusManager::GetFocusedElement(xamlRoot) };
             do
             {

src/cascadia/TerminalControl/SearchBoxControl.cpp

@@ -330,10 +330,14 @@ namespace winrt::Microsoft::Terminal::Control::implementation
     // - bool: whether the current focus is on the search box
     bool SearchBoxControl::ContainsFocus()
     {
-        auto focusedElement = Input::FocusManager::GetFocusedElement(this->XamlRoot());
-        if (_focusableElements.count(focusedElement) > 0)
+        // BODGY: It is possible for this to get called with no XAML root. We are unsure why.
+        if (auto root = XamlRoot())
         {
-            return true;
+            auto focusedElement = Input::FocusManager::GetFocusedElement(root);
+            if (_focusableElements.count(focusedElement) > 0)
+            {
+                return true;
+            }
         }
 
         return false;

src/cascadia/UIHelpers/TextMenuFlyout.cpp

@@ -162,7 +162,13 @@ namespace winrt::Microsoft::Terminal::UI::implementation
         IInspectable target = Target();
         if (!target)
         {
-            target = FocusManager::GetFocusedElement(XamlRoot());
+            auto root = XamlRoot();
+            if (!root)
+            {
+                return;
+            }
+            target = FocusManager::GetFocusedElement(root);
+
             if (!target)
             {
                 return;