perf treport: Fix profile builder type narrowing in progress callback

Mypy does not retain the narrowing of an optional local variable inside
the nested progress callback, causing the perf build to fail with:

  python/treport.py:649: error: Item "None" of "ProfileBuilder | None" has no attribute "cancelled" [union-attr]

Check the loading builder before assigning it to profile so the captured
variable is inferred as ProfileBuilder rather than
Optional[ProfileBuilder]. Keep the existing assertion and background
loading behavior.

Fixes: b379b70b7b ("perf treport: Show the profile while it loads")
Assisted-by: Codex:GPT-6.1-Sol
Signed-off-by: James Clark <james.clark@linaro.org>
Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/python/treport.py

@@ -638,8 +638,9 @@ class ReportApp(App):
     @work(thread=True, exclusive=True)
     def load_profile(self) -> None:
         """Build the profile in a thread so the UI stays responsive."""
-        profile = self.loading
-        assert profile is not None
+        loading = self.loading
+        assert loading is not None
+        profile = loading
 
         def progress() -> None:
             try: