schematic: avoid crash on missing tool

(cherry picked from commit adde48010e)

Fixes https://gitlab.com/kicad/code/kicad/-/issues/21454

eeschema/sch_base_frame.cpp

@@ -776,3 +776,12 @@ void SCH_BASE_FRAME::OnSymChangeDebounceTimer( wxTimerEvent& aEvent )
 
     m_inSymChangeTimerEvent = false;
 }
+
+
+SCH_SELECTION_TOOL* SCH_BASE_FRAME::GetSelectionTool()
+{
+    if( m_toolManager )
+        return m_toolManager->GetTool<SCH_SELECTION_TOOL>();
+
+    return nullptr;
+}

eeschema/sch_base_frame.h

@@ -267,6 +267,8 @@ public:
         m_watcherLastModified = aTime;
     }
 
+    SCH_SELECTION_TOOL* GetSelectionTool() override;
+
 protected:
     void handleActivateEvent( wxActivateEvent& aEvent ) override;