fix(flow): Fix main.dev flow to avoid possibly-null access

app/main.dev.js

@@ -151,6 +151,10 @@ app.setAsDefaultProtocolClient('lightning')
 app.on('open-url', function (event, url) {
   event.preventDefault()
 
+  if (!mainWindow) {
+    throw new Error('"mainWindow" is not defined')
+  }
+
   const payreq = url.split(':')[1]
   mainWindow.webContents.send('lightningPaymentUri', { payreq })
   mainWindow.show()
@@ -196,7 +200,7 @@ export const startLnd = () => {
     }
 
     // Pass current clock height progress to front end for loading state UX
-    if (line.includes('Caught up to height') || line.includes('Catching up block hashes to height')) {
+    if (mainWindow && (line.includes('Caught up to height') || line.includes('Catching up block hashes to height'))) {
       // const blockHeight = line.slice(line.indexOf('Caught up to height') + 'Caught up to height'.length).trim()
       const blockHeight = line.slice(line.indexOf('Caught up to height') + 'Caught up to height'.length).trim()
       mainWindow.webContents.send('lndStdout', line)
@@ -234,10 +238,12 @@ const sendLndSyncing = () => {
     if (didFinishLoad) {
       clearInterval(sendLndSyncingInterval)
 
+      if (mainWindow) {
         console.log('SENDING SYNCING')
         startedSync = true
         mainWindow.webContents.send('lndSyncing')
       }
+    }
   }, 1000)
 }
 
@@ -247,9 +253,11 @@ const sendLndSynced = () => {
     if (didFinishLoad && startedSync) {
       clearInterval(sendLndSyncedInterval)
 
+      if (mainWindow) {
         console.log('SENDING SYNCED')
         mainWindow.webContents.send('lndSynced')
       }
+    }
   }, 1000)
 }
 
@@ -259,9 +267,11 @@ const sendGrpcDisconnected = () => {
     if (didFinishLoad) {
       clearInterval(sendGrpcDisonnectedInterval)
 
+      if (mainWindow) {
         sentGrpcDisconnect = true
         mainWindow.webContents.send('grpcDisconnected')
       }
+    }
   }, 1000)
 }
 
@@ -271,7 +281,9 @@ const sendGrpcConnected = () => {
     if (didFinishLoad && sentGrpcDisconnect) {
       clearInterval(sendGrpcConnectedInterval)
 
+      if (mainWindow) {
         mainWindow.webContents.send('grpcConnected')
       }
+    }
   }, 1000)
 }