fix mouse position bug when canvas.renderMode == ScreenSpaceCamera (https://answers.unity.com/questions/849117/46-ui-image-follow-mouse-position.html)

Runtime/engine/UIWidgetsPanel.cs

 
         void handleMouseMovement() {
             var pos = this.getPointPosition(Input.mousePosition);
+            if (pos == null) {
+                return;
+            }
-                physicalX: pos.x,
-                physicalY: pos.y
+                physicalX: pos.Value.x,
+                physicalY: pos.Value.y
             ));
         }
 
                 var pos = this.getPointPosition(Input.mousePosition);
+                if (pos == null) {
+                    return;
+                }
-                    pos.x,
-                    pos.y,
+                    pos.Value.x,
+                    pos.Value.y,
                     InputUtils.getScrollButtonKey());
             }
         }
         }
 
         public void OnPointerDown(PointerEventData eventData) {
-            EventSystem.current.SetSelectedGameObject(this.gameObject, eventData);
+            if (position == null) {
+                return;
+            }
+            EventSystem.current.SetSelectedGameObject(this.gameObject, eventData);
-                physicalX: position.x,
-                physicalY: position.y
+                physicalX: position.Value.x,
+                physicalY: position.Value.y
+            if (position == null) {
+                return;
+            }
-                physicalX: position.x,
-                physicalY: position.y
+                physicalX: position.Value.x,
+                physicalY: position.Value.y
-        public Vector2 getPointPosition(PointerEventData eventData) {
+        public Vector2? getPointPosition(PointerEventData eventData) {
+            if (eventData.enterEventCamera == null && this.canvas.renderMode == RenderMode.ScreenSpaceCamera) {
+                return null;
+            }
+            
             Vector2 localPoint;
             RectTransformUtility.ScreenPointToLocalPointInRectangle(this.rectTransform, eventData.position,
                 eventData.enterEventCamera, out localPoint);
             return localPoint;
         }
 
-        public Vector2 getPointPosition(Vector2 position) {
+        public Vector2? getPointPosition(Vector2 position) {
             Vector2 localPoint;
             Camera eventCamera = null;
 
 
+            if (eventCamera == null && this.canvas.renderMode == RenderMode.ScreenSpaceCamera) {
+                return null;
+            }
 
             RectTransformUtility.ScreenPointToLocalPointInRectangle(this.rectTransform, position,
                 eventCamera, out localPoint);
 
         public void OnDrag(PointerEventData eventData) {
             var position = this.getPointPosition(eventData);
+            if (position == null) {
+                return;
+            }
-                physicalX: position.x,
-                physicalY: position.y
+                physicalX: position.Value.x,
+                physicalY: position.Value.y
+            var position = this.getPointPosition(eventData);
+            if (position == null) {
+                return;
+            }
-            var position = this.getPointPosition(eventData);
-                physicalX: position.x,
-                physicalY: position.y
+                physicalX: position.Value.x,
+                physicalY: position.Value.y
+            var position = this.getPointPosition(eventData);
+            if (position == null) {
+                return;
+            }
-
-            var position = this.getPointPosition(eventData);
-                physicalX: position.x,
-                physicalY: position.y
+                physicalX: position.Value.x,
+                physicalY: position.Value.y
             ));
         }