Fix.

library/src/main/java/com/afollestad/materialdialogs/MaterialDialog.java

@@ -213,7 +213,7 @@ public class MaterialDialog extends DialogBase implements View.OnClickListener,
             icon.setVisibility(View.VISIBLE);
             icon.setImageDrawable(builder.icon);
         } else {
-            Drawable d = DialogUtils.resolveDrawable(getContext(), R.attr.md_icon);
+            Drawable d = DialogUtils.resolveDrawable(mContext, R.attr.md_icon);
             if (d != null) {
                 icon.setVisibility(View.VISIBLE);
                 icon.setImageDrawable(builder.icon);

sample/src/main/res/values/styles.xml

@@ -5,6 +5,8 @@
         <item name="colorPrimary">@color/material_indigo_500</item>
         <item name="colorPrimaryDark">@color/material_indigo_600</item>
         <item name="colorAccent">@color/material_pink_500</item>
+
+        <item name="md_icon">@drawable/ic_launcher</item>
     </style>
 
 </resources>