Check if workspace is valid during startup (fixes #34)

src/main/kotlin/fxGui/Main.kt

@@ -65,7 +65,22 @@ internal class Main : Application() {
 
         val workspace = if ("workspace" in Config) {
             // Load workspace from config
-            File(Config.workspace)
+            var ws = File(Config.workspace)
+            if (!ws.exists() || !ws.isDirectory) {
+                // The stored path is not valid, choose a new one.
+                PseucoAlert(Alert.AlertType.WARNING).apply {
+                    title = "Invalid workspace"
+                    contentText = "The stored workspace '${ws.absolutePath}' does not exist. Please choose a new one."
+                }.showAndWait()
+                ws = DirectoryChooser().apply {
+                    title = "Update workspace"
+                    initialDirectory = Workspace.DEFAULT
+                }.showDialog(primaryStage)?.also {
+                    Config.workspace = it.absolutePath
+                    Config.save()
+                } ?: exitProcess(0)
+            }
+            ws
         } else {
             // Choose workspace
             DirectoryChooser().apply {