GP: Fix crash when select vertex group in modifiers

source/blender/gpencil_modifiers/intern/MOD_gpencil_util.c

@@ -127,7 +127,7 @@ float get_modifier_point_weight(MDeformVert *dvert, bool inverse, int def_nr)
 {
 	float weight = 1.0f;
 
-	if (def_nr != -1) {
+	if ((dvert != NULL) && (def_nr != -1)) {
 		MDeformWeight *dw = defvert_find_index(dvert, def_nr);
 		weight = dw ? dw->weight : -1.0f;
 		if ((weight >= 0.0f) && (inverse == 1)) {
@@ -145,6 +145,16 @@ float get_modifier_point_weight(MDeformVert *dvert, bool inverse, int def_nr)
 
 	}
 
+	/* handle special empty groups */
+	if ((dvert == NULL) && (def_nr != -1)) {
+		if (inverse == 1) {
+			return 1.0f;
+		}
+		else {
+			return -1.0f;
+		}
+	}
+
 	return weight;
 }
 

source/blender/gpencil_modifiers/intern/MOD_gpencilthick.c

@@ -108,7 +108,7 @@ static void deformStroke(
 
 	for (int i = 0; i < gps->totpoints; i++) {
 		bGPDspoint *pt = &gps->points[i];
-		MDeformVert *dvert = &gps->dvert[i];
+		MDeformVert *dvert = gps->dvert != NULL ? &gps->dvert[i] : NULL;
 		float curvef = 1.0f;
 		/* verify vertex group */
 		const float weight = get_modifier_point_weight(dvert, (mmd->flag & GP_THICK_INVERT_VGROUP) != 0, def_nr);