Added guard around IMSI parameter.

Bug: 22388499
Change-Id: If68165b8d89f369d83c9d6d19896d1802805cef7
diff --git a/service/java/com/android/server/wifi/ConfigurationMap.java b/service/java/com/android/server/wifi/ConfigurationMap.java
index 2a277e7..0fb56ea 100644
--- a/service/java/com/android/server/wifi/ConfigurationMap.java
+++ b/service/java/com/android/server/wifi/ConfigurationMap.java
@@ -52,7 +52,9 @@
                         config.roamingConsortiumIds[i] = id;
                         i++;
                     }
-                    config.enterpriseConfig.setPlmn(homeSp.getCredential().getImsi().toString());
+                    IMSIParameter imsiParameter = homeSp.getCredential().getImsi();
+                    config.enterpriseConfig.setPlmn(
+                            imsiParameter != null ? imsiParameter.toString() : null);
                     config.enterpriseConfig.setRealm(homeSp.getCredential().getRealm());
                     mPerFQDN.put(fqdn, config.networkId);
                 }