Bug #143. Fix NPE in HeatMapPanel.

Author: mikekucera

EnrichmentMapPlugin/src/main/java/org/baderlab/csplugins/enrichmentmap/view/HeatMapPanel.java

@@ -1178,7 +1178,7 @@ private JPanel createExpressionLegendPanel() {
 		layout.setHorizontalGroup(hGroup);
 		layout.setVerticalGroup(vGroup);
 
-		if (this.currentExpressionSet2 != null && !this.currentExpressionSet2.isEmpty()) {
+		if (params.isTwoDistinctExpressionSets()) {
 			ColorGradientWidget legend1 = ColorGradientWidget.getInstance("", hmParams.getTheme_ds1(),
 					hmParams.getRange_ds1(), true, ColorGradientWidget.LEGEND_POSITION.NA);
 			ColorGradientWidget legend2 = ColorGradientWidget.getInstance("", hmParams.getTheme_ds2(),