Don't attempt to split settings tabs (#13172)

Prevents a null pointer dereference when attempting to split a settings tab, due to it not being a terminal tab.

* [x] Closes #13166

## Validation Steps Performed
Manually tested.

src/cascadia/TerminalApp/TerminalPage.cpp

@@ -1822,9 +1822,17 @@ namespace winrt::TerminalApp::implementation
         // Instead, let's just promote this first split to be a tab instead.
         // Crash avoided, and we don't need to worry about inserting a new-tab
         // command in at the start.
-        if (!focusedTab && _tabs.Size() == 0)
+        if (!focusedTab)
         {
-            _CreateNewTabFromPane(newPane);
+            if (_tabs.Size() == 0)
+            {
+                _CreateNewTabFromPane(newPane);
+            }
+            else
+            {
+                // The focused tab isn't a terminal tab
+                return;
+            }
         }
         else
         {