diff --git a/test/test_mainwindow.py b/test/test_mainwindow.py
index b16d85d..d43771d 100644
--- a/test/test_mainwindow.py
+++ b/test/test_mainwindow.py
@@ -114,3 +114,14 @@ def check_file_format(filename: str) -> None:
     check_file_format("demo.zxg")
     check_file_format("demo.zxp")
     check_file_format("demo.zxr")
+
+
+def test_proof_cleanup_before_close(app: MainWindow, qtbot: QtBot) -> None:
+    # Regression test to check that the app doesn't crash when closing a proof tab with a derivation in progress,
+    # due to accessing the graph after it has been deallocated.
+    # See https://github.com/Quantomatic/zxlive/issues/218 for context.
+    assert app.active_panel is not None
+    assert isinstance(app.active_panel, GraphEditPanel)
+    qtbot.mouseClick(app.active_panel.start_derivation, QtCore.Qt.MouseButton.LeftButton)
+    app.select_all_action.trigger()
+    app.close_action.trigger()
diff --git a/zxlive/mainwindow.py b/zxlive/mainwindow.py
index 006783c..7e06b5c 100644
--- a/zxlive/mainwindow.py
+++ b/zxlive/mainwindow.py
@@ -308,7 +308,8 @@ def close_tab(self, i: int) -> bool:
         if i == -1:
             return False
         widget = self.tab_widget.widget(i)
-        if isinstance(widget, BasePanel) and not widget.undo_stack.isClean():
+        assert isinstance(widget, BasePanel)
+        if not widget.undo_stack.isClean():
             name = self.tab_widget.tabText(i).replace("*","")
             answer = QMessageBox.question(self, "Save Changes",
                             f"Do you wish to save your changes to {name} before closing?",
@@ -320,6 +321,7 @@ def close_tab(self, i: int) -> bool:
                 val = self.handle_save_file_action()
                 if not val:
                     return False
+        widget.graph_scene.clearSelection()
         self.tab_widget.removeTab(i)
         if self.tab_widget.count() == 0:
             self._reset_menus(False)