Fix NullPointerException triggered by calling imgLabeling.getType().c…

…lear()

The bug was discovered by compiling and testing Labkit against
imglib2-7.1. Trying to use the brush in Labkit would trigger this
NullPointerException.

src/main/java/net/imglib2/roi/labeling/ImgLabeling.java

@@ -243,7 +243,7 @@ public LabelingType< T > firstElement()
 	@Override
 	public LabelingType< T > getType()
 	{
-		return new LabelingType<>( null, mapping, generation );
+		return new LabelingType<>( indexAccessible.getType(), mapping, generation );
 	}
 
 	@Override

src/test/java/net/imglib2/roi/labeling/ImgLabelingTest.java

@@ -34,6 +34,7 @@
 package net.imglib2.roi.labeling;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 
 import java.util.Arrays;
@@ -174,4 +175,14 @@ public void testHashCodeAndEquals()
 		assertTrue( pixel.equals( expected ) );
 		assertEquals( expected.hashCode(), pixel.hashCode() );
 	}
+
+	@Test
+	public void testGetType()
+	{
+		ImgLabeling< String, IntType > imgLabeling = new ImgLabeling<>( ArrayImgs.ints( 10, 10 ) );
+		LabelingType< String > type = imgLabeling.getType().createVariable();
+		type.clear();
+		type.add( "A" );
+		assertEquals( Collections.singleton( "A" ), type );
+	}
 }