Fix NullPointerException on RingView

app/src/main/java/org/isoron/uhabits/views/RingView.java

@@ -27,6 +27,7 @@ import android.graphics.Paint;
 import android.graphics.PorterDuff;
 import android.graphics.PorterDuffXfermode;
 import android.graphics.RectF;
+import android.support.annotation.Nullable;
 import android.text.TextPaint;
 import android.util.AttributeSet;
 import android.view.View;
@@ -57,8 +58,10 @@ public class RingView extends View
     private float textSize;
     private boolean enableFontAwesome;
 
+    @Nullable
     private Bitmap drawingCache;
     private Canvas cacheCanvas;
+
     private boolean isTransparencyEnabled;
 
     public RingView(Context context)
@@ -175,13 +178,15 @@ public class RingView extends View
     {
         super.onSizeChanged(w, h, oldw, oldh);
 
-        if(isTransparencyEnabled)
+        if(isTransparencyEnabled) reallocateCache();
+    }
+
+    private void reallocateCache()
     {
         if (drawingCache != null) drawingCache.recycle();
         drawingCache = Bitmap.createBitmap(diameter, diameter, Bitmap.Config.ARGB_8888);
         cacheCanvas = new Canvas(drawingCache);
     }
-    }
 
     @Override
     protected void onDraw(Canvas canvas)
@@ -191,6 +196,7 @@ public class RingView extends View
 
         if(isTransparencyEnabled)
         {
+            if(drawingCache == null) reallocateCache();
             activeCanvas = cacheCanvas;
             drawingCache.eraseColor(Color.TRANSPARENT);
         }