allow build apk when key.properties file is missing

Author: X-Wei

android/app/build.gradle

@@ -56,21 +56,25 @@ android {
     }
 
     signingConfigs {
-    release {
-      def keystoreProperties = new Properties()
-      def keystorePropertiesFile = rootProject.file('key.properties')
-      if (keystorePropertiesFile.exists()) {
-          keystoreProperties.load(new FileInputStream(keystorePropertiesFile))
-          keyAlias keystoreProperties['keyAlias']
-          keyPassword keystoreProperties['keyPassword']
-          storeFile file(keystoreProperties['storeFile'])
-          storePassword keystoreProperties['storePassword']
-      }
-    }
+        release {
+            def keystoreProperties = new Properties()
+            def keystorePropertiesFile = rootProject.file('key.properties')
+            if (keystorePropertiesFile.exists()) {
+                keystoreProperties.load(new FileInputStream(keystorePropertiesFile))
+                keyAlias keystoreProperties['keyAlias']
+                keyPassword keystoreProperties['keyPassword']
+                storeFile file(keystoreProperties['storeFile'])
+                storePassword keystoreProperties['storePassword']
+            }
+        }
     }
     buildTypes {
         release {
-            signingConfig signingConfigs.release
+            if (signingConfigs.release.storeFile) {
+                signingConfig signingConfigs.release
+            } else {
+                signingConfig signingConfigs.debug
+            }
         }
     }
 }