Draft: fix constrain error if there is no new point (#26868)

* Add None check before constraining new_point

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/Mod/Draft/DraftGui.py

@@ -1719,7 +1719,10 @@ class DraftToolBar:
             self.mask = val
             if hasattr(FreeCADGui, "Snapper"):
                 FreeCADGui.Snapper.mask = val
-                self.new_point = FreeCADGui.Snapper.constrain(self.new_point, self.get_last_point())
+                if self.new_point is not None:
+                    self.new_point = FreeCADGui.Snapper.constrain(
+                        self.new_point, self.get_last_point()
+                    )
 
     def changeXValue(self, d):
         if self.display_point_active: