fix: resolve custom threshold configuration errors

- Add validation to prevent API calls with missing nodeId/vmid parameters
- Implement proper guest selector initialization for edit modal
- Add server-side error handlers to return JSON instead of HTML for API routes
- Include debug logging for threshold configuration troubleshooting
- Ensure dropdown selection properly populates all required hidden fields

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

server/index.js

@@ -723,6 +723,31 @@ app.get('/api/diagnostics', async (req, res) => {
     }
 });
 
+// Global error handler for unhandled API errors
+app.use((err, req, res, next) => {
+    console.error('Unhandled API error:', err);
+    
+    // Ensure we always return JSON for API routes
+    if (req.url.startsWith('/api/')) {
+        return res.status(500).json({
+            success: false,
+            error: 'Internal server error',
+            message: err.message
+        });
+    }
+    
+    // For non-API routes, return HTML error
+    res.status(500).send('Internal Server Error');
+});
+
+// 404 handler for API routes
+app.use('/api/*', (req, res) => {
+    res.status(404).json({
+        success: false,
+        error: 'API endpoint not found'
+    });
+});
+
 // --- WebSocket Setup ---
 const io = new Server(server, {
   // Optional: Configure CORS for Socket.IO if needed, separate from Express CORS

src/public/js/ui/settings.js

@@ -1440,6 +1440,16 @@ PulseApp.ui.settings = (() => {
         const nodeField = document.getElementById('threshold-node');
         const vmidField = document.getElementById('threshold-vmid');
         
+        // If editing existing threshold, select the correct guest in dropdown
+        if (endpointId && vmid) {
+            const selectValue = `${endpointId}:${vmid}`;
+            guestSelector.value = selectValue;
+            
+            // Trigger change event to populate hidden fields properly
+            const changeEvent = new Event('change');
+            guestSelector.dispatchEvent(changeEvent);
+        }
+        
         guestSelector.addEventListener('change', (e) => {
             if (e.target.value) {
                 const [selectedEndpoint, selectedVmid] = e.target.value.split(':');
@@ -1449,6 +1459,14 @@ PulseApp.ui.settings = (() => {
                 // Find the current node for this VM (for display purposes)
                 const selectedGuest = allGuests.find(g => g.endpointId === selectedEndpoint && g.id === selectedVmid);
                 nodeField.value = selectedGuest ? selectedGuest.node : '';
+                
+                // Debug logging
+                console.log('[Settings] Guest selector changed:', {
+                    selectedEndpoint,
+                    selectedVmid,
+                    selectedGuest,
+                    nodeValue: selectedGuest ? selectedGuest.node : 'MISSING'
+                });
             } else {
                 endpointField.value = '';
                 nodeField.value = '';
@@ -1496,6 +1514,14 @@ PulseApp.ui.settings = (() => {
             const nodeId = document.getElementById('threshold-node').value;
             const vmid = document.getElementById('threshold-vmid').value;
             
+            // Validate required fields
+            if (!endpointId || !nodeId || !vmid) {
+                alert('Please select a VM/LXC from the dropdown first.');
+                return;
+            }
+            
+            console.log('[Settings] Saving thresholds for:', { endpointId, nodeId, vmid });
+            
             try {
                 const method = existingThresholds ? 'PUT' : 'POST';
                 const response = await fetch(`/api/thresholds/${endpointId}/${nodeId}/${vmid}`, {