Handle error case when no command found

Fix for #25

src/mongo/commands.ts

@@ -22,8 +22,13 @@ export class MongoCommands {
 		}
 		const selection = activeEditor.selection;
 		const command = MongoCommands.getCommand(activeEditor.document.getText(), selection.start);
-		MongoCommands.executeCommand(command, database)
-			.then(result => this.showResult(result, activeEditor.viewColumn + 1));
+		if (command) {
+			MongoCommands.executeCommand(command, database)
+				.then(result => this.showResult(result, activeEditor.viewColumn + 1));
+		} else {
+			vscode.window.showErrorMessage('No executable command found.');
+		}
+
 		return command;
 	}