Fix sonar issues

Author: nroduit

weasis-dicom/weasis-dicom-viewer2d/src/main/java/org/weasis/dicom/viewer2d/mpr/cmpr/CurvedMprAxis.java

@@ -78,7 +78,7 @@ public DebugCurveData(
   private CurvedMprImageIO io;
   private DicomImageElement imageElement;
   private CurvedMprView view;
-  private volatile DebugCurveData debugData;
+  private volatile DebugCurveData debugData; // NOSONAR guarantees visibility of the reference
 
   // Live-edit binding to the source polyline. Optional.
   private MprView sourceView;

weasis-dicom/weasis-dicom-viewer2d/src/main/java/org/weasis/dicom/viewer2d/mpr/cmpr/CurvedMprView.java

@@ -202,9 +202,17 @@ private void addStepSlider(JPanel panel, double pixelMm, String unitLabel) {
   }
 
   private String unitAbbreviation() {
-    DicomImageElement img = curvedMprAxis == null ? null : curvedMprAxis.getImageElement();
-    Unit unit = img == null ? Unit.MILLIMETER : img.getPixelSpacingUnit();
-    return unit == null ? Unit.MILLIMETER.getAbbreviation() : unit.getAbbreviation();
+    Unit unit = Unit.MILLIMETER;
+    if (curvedMprAxis != null) {
+      DicomImageElement img = curvedMprAxis.getImageElement();
+      if (img != null) {
+        Unit spacingUnit = img.getPixelSpacingUnit();
+        if (spacingUnit != null) {
+          unit = spacingUnit;
+        }
+      }
+    }
+    return unit.getAbbreviation();
   }
 
   /** Fall back to 1.0 if the volume reports a degenerate (0 or negative) pixel spacing. */