Fix NPE in drawChild() when returning to Fragment with animation. Appears very similar to Issue umano#599 in umano/AndroidSlidingUpPanel. Put in a check for mSlideableView != null

Author: pandasys

library/src/main/java/com/sothree/slidinguppanel/SlidingUpPanelLayout.java

@@ -1154,7 +1154,7 @@ protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
         boolean result;
         final int save = canvas.save(Canvas.CLIP_SAVE_FLAG);
 
-        if (mSlideableView != child) { // if main view
+        if (mSlideableView != null && mSlideableView != child) { // if main view
             // Clip against the slider; no sense drawing what will immediately be covered,
             // Unless the panel is set to overlay content
             canvas.getClipBounds(mTmpRect);