added support to consider DOT.exe location from 'GRAPHVIZ_DOT' environment variable

diff --git a/eclipse-tools/emf-graphical-viewer/plugins/org.eclipse.app4mc.emf.viewer.plantuml/src/org/eclipse/app4mc/emf/viewer/plantuml/handlers/AbstractPlantUMLHandler.java b/eclipse-tools/emf-graphical-viewer/plugins/org.eclipse.app4mc.emf.viewer.plantuml/src/org/eclipse/app4mc/emf/viewer/plantuml/handlers/AbstractPlantUMLHandler.java
index 3de0a86..036126d 100644
--- a/eclipse-tools/emf-graphical-viewer/plugins/org.eclipse.app4mc.emf.viewer.plantuml/src/org/eclipse/app4mc/emf/viewer/plantuml/handlers/AbstractPlantUMLHandler.java
+++ b/eclipse-tools/emf-graphical-viewer/plugins/org.eclipse.app4mc.emf.viewer.plantuml/src/org/eclipse/app4mc/emf/viewer/plantuml/handlers/AbstractPlantUMLHandler.java

@@ -88,7 +88,7 @@
 			EObject eObject) {
 		
 		// check if there is a valid dot path
-		if (verifyDotPath(shell, dotPath)) {
+		if (verifyDotPath(shell, sync, dotPath)) {
 			
 			// generate UML
 			if (selected != null) {
@@ -130,20 +130,60 @@
 			}
 		}
 	}
-	
-	protected boolean verifyDotPath(Shell shell, String dotPath) {
-		if (dotPath == null) {
-			openErrorDialog(shell, "Missing Graphviz DOT.exe location in Preferences."+System.getProperty("line.separator")+"Please specify location via \"Window - Preferences - PlantUML - Path to the dot executable of Graphviz.\"");
-			return false;
-		} else if (!new File(dotPath).exists()) {
-			openErrorDialog(shell, "Invalid path of Graphviz DOT.exe is present in Preferences."+System.getProperty("line.separator")+"Please specify valid path via \"Window - Preferences - PlantUML - Path to the dot executable of Graphviz.\"");
-			return false;
+
+	protected boolean verifyDotPath(Shell shell, UISynchronize sync, String dotPathFromPlantUMLPreferencePage) {
+
+		String dotPathFromSystemProperty = System.getenv("GRAPHVIZ_DOT");
+
+		File dotFile = null;
+
+		if (dotPathFromPlantUMLPreferencePage != null) {
+			dotFile = ensureValidDotFile(shell, sync, dotPathFromPlantUMLPreferencePage, "PlantUMLPreference");
+		} else if (dotPathFromSystemProperty != null) {
+			dotFile = ensureValidDotFile(shell, sync, dotPathFromSystemProperty, "SystemProperty");
+		} else {
+			showErrorDialog(shell, sync, "Missing Graphviz dot.exe location."
+					+ "\nPlease specify location via Window - Preferences - PlantUML - Path to the dot executable of Graphviz.");
 		}
 
-		System.setProperty("GRAPHVIZ_DOT", dotPath);
+		if (dotFile == null)
+			return false;
+
+		System.setProperty("GRAPHVIZ_DOT", dotFile.getAbsolutePath());
 
 		return true;
 	}
+
+	private File ensureValidDotFile(Shell shell, UISynchronize sync, String dotPath, String origin) {
+		
+		
+		if (dotPath == null || dotPath.equals("")) {
+			showErrorDialog(shell, sync, "Missing Graphviz dot.exe location."
+						+ "\nPlease specify location via Window - Preferences - PlantUML - Path to the dot executable of Graphviz.");
+			return null;
+		}
+
+		final File dotFile = new File(dotPath);
+		if (!dotFile.canExecute()) {
+			
+			if(origin.equals("PlantUMLPreference")) {
+				showErrorDialog(shell, sync, "Invalid Graphviz dot.exe location: \'" + dotFile.getAbsolutePath()+"\'"
+				+ "\nPlease set location via Window - Preferences - PlantUML - Path to the 'dot executable of Graphviz'.");
+			}else {
+				showErrorDialog(shell, sync, "Invalid Graphviz dot.exe location found in System Property 'GRAPHVIZ_DOT' : \'" + dotFile.getAbsolutePath()+"\'"
+				+ "\nPlease update the System Property 'GRAPHVIZ_DOT' with valid Path to the 'dot executable of Graphviz'.");
+			}
+			return null;
+		}
+
+		return dotFile;
+	}
+
+	private void showErrorDialog(Shell shell, UISynchronize sync, String message) {
+		sync.asyncExec(() ->
+			MessageDialog.openError(shell, "AMALTHEA HW Visualization", message)
+		);
+	}
 	
 	protected void openErrorDialog(Shell shell) {
 		openErrorDialog(shell, "Selected object is not a EMF object");