Refactor

android/build.gradle

+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
 apply plugin: 'com.android.library'
 
 def DEFAULT_COMPILE_SDK_VERSION   = 26
@@ -6,12 +10,12 @@ def DEFAULT_TARGET_SDK_VERSION    = 26
 def DEFAULT_MIN_SDK_VERSION       = 16
 
 android {
-        compileSdkVersion rootProject.hasProperty('compileSdkVersion') ? rootProject.compileSdkVersion : DEFAULT_COMPILE_SDK_VERSION
-        buildToolsVersion rootProject.hasProperty('buildToolsVersion') ? rootProject.buildToolsVersion : DEFAULT_BUILD_TOOLS_VERSION
+        compileSdkVersion safeExtGet('compileSdkVersion', 26)
+        buildToolsVersion safeExtGet('buildToolsVersion', '26.0.3')
 
   defaultConfig {
-        minSdkVersion rootProject.hasProperty('minSdkVersion') ? rootProject.minSdkVersion : DEFAULT_MIN_SDK_VERSION
-        targetSdkVersion rootProject.hasProperty('targetSdkVersion') ? rootProject.targetSdkVersion : DEFAULT_TARGET_SDK_VERSION
+        minSdkVersion safeExtGet('minSdkVersion', 16)
+        targetSdkVersion safeExtGet('targetSdkVersion', 26)
         versionCode 1
         versionName "1.0"
         ndk {