Fix Headless exceptions

Author: tferr

src/main/java/sc/fiji/snt/SNT.java

@@ -633,13 +633,7 @@ private void nullifyCanvases(final boolean disposeXY) {
 	}
 
 	public boolean accessToValidImageData() {
-		// NB: Currently we are assuming that image data comes from an ImagePlus. This
-		// needs to be changed when ImagePlus requirements are lift. Note that xy may
-		// be null (e.g., image has been manually closed), but its cached data ctSlice3d
-		// may still be available. Return ctSlice3d != null is problematic on its own,
-		// because there are several calls to this method that happen _before_ ctSlice3d
-		// has been assembled, but _after_ an image has been specified
-		return getImagePlus() != null && !isDisplayCanvas(xy);
+		return (getImagePlus() != null && !isDisplayCanvas(xy)) || ctSlice3d != null;
 	}
 
 	private void setIsDisplayCanvas(final ImagePlus imp) {
@@ -839,7 +833,7 @@ public void rebuildZYXZpanes() {
 		if (!xz.isVisible()) xz.show();
 	}
 
-	@SuppressWarnings("unchecked")
+    @SuppressWarnings("unchecked")
 	private void loadDatasetFromImagePlus(final ImagePlus imp) {
 		statusService.showStatus("Loading data...");
 		this.ctSlice3d = ImgUtils.getCtSlice3d(imp, channel - 1, frame - 1);

src/main/java/sc/fiji/snt/hyperpanes/MultiDThreePanes.java

@@ -22,7 +22,7 @@
 
 package sc.fiji.snt.hyperpanes;
 
-import java.awt.Color;
+import java.awt.*;
 import java.awt.image.ColorModel;
 
 import ij.ImagePlus;
@@ -664,6 +664,7 @@ private void initialize(final int frame) {
 	}
 
 	private void initCanvasesAndWindows() {
+        if (GraphicsEnvironment.isHeadless()) return;
 		xy_canvas = createCanvas(xy, XY_PLANE);
 		if (isDummy()) {
 			xy_window = null;