fix nil map error when using plugin headers in legacy format (#3996)

* fix nil map error when using plugin headers in legacy format

* create map on demand

* better initialization

README.md

@@ -201,11 +201,11 @@ This example implements multiple SSH services exposed through the same port usin
 
 4. To access internal machine A using SSH ProxyCommand, assuming the username is "test":
 
-  `ssh -o 'proxycommand socat - PROXY:x.x.x.x:machine-a.example.com:22,proxyport=5002' test@machine-a`
+  `ssh -o 'proxycommand socat - PROXY:x.x.x.x:%h:%p,proxyport=5002' test@machine-a.example.com`
 
 5. To access internal machine B, the only difference is the domain name, assuming the username is "test":
 
-  `ssh -o 'proxycommand socat - PROXY:x.x.x.x:machine-b.example.com:22,proxyport=5002' test@machine-b`
+  `ssh -o 'proxycommand socat - PROXY:x.x.x.x:%h:%p,proxyport=5002' test@machine-b.example.com`
 
 ### Accessing Internal Web Services with Custom Domains in LAN
 
@@ -534,6 +534,8 @@ webServer.password = "admin"
 
 Then visit `http://[serverAddr]:7500` to see the dashboard, with username and password both being `admin`.
 
+Note that if you want your server to be accessed from public networks, then also add `webServer.addr = "0.0.0.0"` line. For security reasons (credits [#3709](https://github.com/fatedier/frp/issues/3709)), value `127.0.0.1` is used by default. 
+
 Additionally, you can use HTTPS port by using your domains wildcard or normal SSL certificate:
 
 ```toml

pkg/config/legacy/conversion.go

@@ -175,6 +175,9 @@ func transformHeadersFromPluginParams(params map[string]string) v1.HeaderOperati
 			continue
 		}
 		if k = strings.TrimPrefix(k, "plugin_header_"); k != "" {
+			if out.Set == nil {
+				out.Set = make(map[string]string)
+			}
 			out.Set[k] = v
 		}
 	}