fix error

src/common/DayGrid.js

@@ -276,7 +276,7 @@ var DayGrid = FC.DayGrid = Grid.extend(DayTableMixin, {
 		this.renderHighlight(this.eventToSpan(eventLocation));
 
 		// if a segment from the same calendar but another component is being dragged, render a helper event
-		if (seg.component !== this) {
+		if (seg && seg.component !== this) {
 			return this.renderEventLocationHelper(eventLocation, seg); // returns mock event elements
 		}
 	},