Fix bug Field 'lastPosition' has not been initialized when pressed - #235

════════ Exception caught by gesture ═══════════════════════════════════════════
The following LateError was thrown while handling a gesture:
LateInitializationError: Field 'lastPosition' has not been initialized.

When the exception was thrown, this was the stack
#0      _SlidableGestureDetectorState.lastPosition (package:flutter_slidable/src/gesture_detector.dart)
package:flutter_slidable/src/gesture_detector.dart:1
#1      _SlidableGestureDetectorState.handleDragEnd
package:flutter_slidable/src/gesture_detector.dart:93
#2      DragGestureRecognizer._checkEnd.<anonymous closure>
package:flutter/…/gestures/monodrag.dart:473
#3      GestureRecognizer.invokeCallback
package:flutter/…/gestures/recognizer.dart:182
#4      DragGestureRecognizer._checkEnd
package:flutter/…/gestures/monodrag.dart:473
...
Handler: "onEnd"
Recognizer: HorizontalDragGestureRecognizer#c5a3b
    debugOwner: GestureDetector
    start behavior: down
════════════════════════════════════════════════════════════════════════════════
Application finished.