Show error dialog if there is no PythonScriptRunner

Currently the PythonScriptRunner is not integrated into the ImageJ
Launcher so it is only available if Fiji is launched from python and it
is inserted in the ObjectService. Now we provide an error message if it
is not found in the ObjectService.

Author: karlduderstadt

src/main/java/org/scijava/plugins/scripting/python/PythonScriptEngine.java

@@ -37,14 +37,16 @@
 import java.util.Set;
 
 import javax.script.Bindings;
-import javax.script.ScriptException;
 import javax.script.ScriptEngine;
-import org.scijava.script.AbstractScriptEngine;
+import javax.script.ScriptException;
+
 import org.scijava.Context;
 import org.scijava.log.LogService;
 import org.scijava.object.ObjectService;
 import org.scijava.plugin.Parameter;
-import org.scijava.plugins.scripting.python.PythonScriptRunner;
+import org.scijava.script.AbstractScriptEngine;
+import org.scijava.ui.DialogPrompt;
+import org.scijava.ui.UIService;
 
 /**
  * A script engine for conda-based python.
@@ -60,6 +62,9 @@ public class PythonScriptEngine extends AbstractScriptEngine {
 
 	@Parameter
 	LogService logService;
+	
+	@Parameter
+	UIService uiService;
 
 	public PythonScriptEngine(Context context) {
 		context.inject(this);
@@ -69,7 +74,13 @@ public PythonScriptEngine(Context context) {
 
 	@Override
 	public Object eval(String script) throws ScriptException {
-		return objectService.getObjects(PythonScriptRunner.class).get(0).run(script, engineScopeBindings, scriptContext);
+		if (objectService.getObjects(PythonScriptRunner.class).stream().count() > 0)
+			return objectService.getObjects(PythonScriptRunner.class).get(0).run(script, engineScopeBindings, scriptContext);
+		
+		uiService.showDialog("The PythonScriptRunner could not be found in the ObjectService. To use the\n" +
+			                   "Conda Python 3 script engine Fiji must be launched from python inside a conda\n " +
+			                   "environment and a PythonScriptRunner must be added to the ObjectService.\n", DialogPrompt.MessageType.ERROR_MESSAGE);
+		return null;
 	}
 
 	@Override