better handling of the case where the chain is (for some reason) behind the wallet tip.

Breeze/src/Breeze.Wallet/LightWalletSyncManager.cs

@@ -37,15 +37,32 @@ namespace Breeze.Wallet
         /// <inheritdoc />
         public Task Initialize()
         {            
-            this.walletTip = this.chain.GetBlock(this.walletManager.WalletTipHash);
-            if (this.walletTip == null)
-                throw new WalletException("the wallet tip was not found in the main chain");
-            
             // if there is no wallet created yet, the wallet tip is the chain tip.
             if (!this.walletManager.Wallets.Any())
             {
                 this.walletTip = this.chain.Tip;
             }
+            else
+            {
+                this.walletTip = this.chain.GetBlock(this.walletManager.WalletTipHash);
+                if (this.walletTip == null)
+                {
+                    // the wallet tip was not found in the main chain.
+                    // this can happen if the node crashes unexpectedly.
+                    // to recover we need to find the first common fork 
+                    // with the best chain, as the wallet does not have a  
+                    // list of chain headers we use a BlockLocator and persist 
+                    // that in the wallet. the block locator will help finding 
+                    // a common fork and bringing the wallet back to a good 
+                    // state (behind the best chain)
+                    var locators = this.walletManager.Wallets.First().BlockLocator;
+                    BlockLocator blockLocator = new BlockLocator { Blocks = locators.ToList() };
+                    var fork = this.chain.FindFork(blockLocator);
+                    this.walletManager.RemoveBlocks(fork);
+                    this.walletManager.WalletTipHash = fork.HashBlock;
+                    this.walletTip = fork;
+                }
+            }
 
             // subscribe to receiving blocks and transactions
             BlockSubscriber sub = new BlockSubscriber(this.signals.Blocks, new BlockObserver(this));