Check for 2D images in imagej plugin

src/ImageJ/plugin-ij/src/main/java/net/leverjs/LeverscIJ.java

@@ -188,6 +188,13 @@ public class LeverscIJ implements PlugInFilter
         // Keep original format around to help with conversion
         imType = image.getType();
 
+        int nslice = image.getNSlices();
+        if ( nslice <= 1 )
+        {
+            IJ.error("Invalid Image", "Leversc does not support 2D images!");;
+            return;
+        }
+
         int fignum = get_figunum();
         if ( fignum <= 0 )
             return;