Fix the renamer tests (microsoft#9815)

## Summary of the Pull Request

Clearly, I didn't run these tests on my last commit where I made the toasts lazy-load.

## References
* broken in in microsoft#9662
* 
## PR Checklist
* [x] Closes microsoft#9769
* [x] I work here
* [x] Tests added/passed
* [n/a] Requires documentation to be updated

## Detailed Description of the Pull Request / Additional comments

For whatever reason, these tests are unhappy running back to back, but are just fine running isolated.

src/cascadia/LocalTests_TerminalApp/TabTests.cpp

@@ -954,6 +954,10 @@ namespace TerminalAppLocalTests
 
     void TabTests::TestWindowRenameSuccessful()
     {
+        BEGIN_TEST_METHOD_PROPERTIES()
+            TEST_METHOD_PROPERTY(L"IsolationLevel", L"Method")
+        END_TEST_METHOD_PROPERTIES()
+
         auto page = _commonSetup();
         page->RenameWindowRequested([&page](auto&&, const winrt::TerminalApp::RenameWindowRequestedArgs args) {
             // In the real terminal, this would bounce up to the monarch and
@@ -980,6 +984,10 @@ namespace TerminalAppLocalTests
     }
     void TabTests::TestWindowRenameFailure()
     {
+        BEGIN_TEST_METHOD_PROPERTIES()
+            TEST_METHOD_PROPERTY(L"IsolationLevel", L"Method")
+        END_TEST_METHOD_PROPERTIES()
+
         auto page = _commonSetup();
         page->RenameWindowRequested([&page](auto&&, auto&&) {
             // In the real terminal, this would bounce up to the monarch and

src/cascadia/TerminalApp/TerminalPage.cpp

@@ -2708,8 +2708,12 @@ namespace winrt::TerminalApp::implementation
     void TerminalPage::_RequestWindowRename(const winrt::hstring& newName)
     {
         auto request = winrt::make<implementation::RenameWindowRequestedArgs>(newName);
-        // The WindowRenamer is _not_ a Toast - we want it to stay open until the user dismisses it.
-        WindowRenamer().IsOpen(false);
+        // The WindowRenamer is _not_ a Toast - we want it to stay open until
+        // the user dismisses it.
+        if (WindowRenamer())
+        {
+            WindowRenamer().IsOpen(false);
+        }
         _RenameWindowRequestedHandlers(*this, request);
         // We can't just use request.Successful here, because the handler might
         // (will) be handling this asynchronously, so when control returns to