throwing in the simplest fix i can think of while i ponder about what…

… the real fix should be (#4559)

## Summary of the Pull Request
The issue seems to be how `SwapChainScaleChanged` gets fired and attempts to tell the renderer
to `UpdateDPI` when the renderer is gone. So, as a quick bandaid, we'll put a quick check to only do the thing if the renderer is alive.

## PR Checklist
* [x] Closes #4539
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [x] Tests added/passed

## Validation Steps Performed
Held my new tab button for about thirty seconds then held the close tab button until all tabs closed without a crash.

src/cascadia/TerminalControl/TermControl.cpp

@@ -1521,12 +1521,15 @@ namespace winrt::Microsoft::Terminal::TerminalControl::implementation
     void TermControl::_SwapChainScaleChanged(Windows::UI::Xaml::Controls::SwapChainPanel const& sender,
                                              Windows::Foundation::IInspectable const& /*args*/)
     {
-        const auto scale = sender.CompositionScaleX();
-        const auto dpi = (int)(scale * USER_DEFAULT_SCREEN_DPI);
+        if (_renderEngine)
+        {
+            const auto scale = sender.CompositionScaleX();
+            const auto dpi = (int)(scale * USER_DEFAULT_SCREEN_DPI);
 
-        // TODO: MSFT: 21169071 - Shouldn't this all happen through _renderer and trigger the invalidate automatically on DPI change?
-        THROW_IF_FAILED(_renderEngine->UpdateDpi(dpi));
-        _renderer->TriggerRedrawAll();
+            // TODO: MSFT: 21169071 - Shouldn't this all happen through _renderer and trigger the invalidate automatically on DPI change?
+            THROW_IF_FAILED(_renderEngine->UpdateDpi(dpi));
+            _renderer->TriggerRedrawAll();
+        }
     }
 
     // Method Description: