Check if module is present before removing it

Apparently not all tests that add modules to the PAGE_REGISTRY also add
them to sys.modules.

Author: rnestler

tests/conftest.py

@@ -23,7 +23,8 @@ def clear_pages_state():
 def init_pages_state():
     """Clear all global state that is used by pages feature."""
     for page in dash._pages.PAGE_REGISTRY.values():
-        sys.modules.pop(page["module"])
+        if page["module"] in sys.modules:
+            sys.modules.pop(page["module"])
     dash._pages.PAGE_REGISTRY.clear()
     dash._pages.CONFIG.clear()
     dash._pages.CONFIG.__dict__.clear()