fixed issue with "empty" elements [walls, stairs, elevators] during tool selection

minor ui changes

mapview/2D/tools/ToolNewWall.h

@@ -15,7 +15,15 @@ private:
 public:
 
 	ToolNewWall(Tools& tools, MapLayer* layer) : ToolNewElement(tools, layer) {
-		create();
+		;
+	}
+
+	void becomesActive() override {
+		create();			// start adding an new element
+	}
+
+	void becomesInactive() override {
+		deleteCurrent();	// delete the currently pending and not yet finished wall element
 	}
 
 	const std::string getName() const override {