Worked on stabilizing the Sketcher module in the FreeCAD/FreeCAD repository by addressing a critical segmentation fault affecting DatumLine operations. The solution involved introducing a null-check for the refSubShape object, ensuring that external geometry projections and intersections only proceed when a valid reference sub-shape is present. This targeted bug fix, implemented in C++, reduced the likelihood of crashes during common sketching scenarios and contributed to more robust CAD workflows. The work demonstrated a focus on software reliability and maintainability, leveraging skills in bug fixing and C++ development to enhance the stability of external geometry interactions within the Sketcher environment.