错误描述：

dyld: Symbol not found: _OBJC_CLASS_$_UIPresentationController

 Referenced from: /var/mobile/Applications/ 2296090E-900A-47DE-9D6F-972CAAE4524F/FamilySchoolPro    copy.app/FamilySchoolPro copy

 Expected in: /System/Library/Frameworks/   UIKit.framework/UIKit

in /var/mobile/Applications/2296090E-900A-47DE-9D6F-972CAAE4524F/FamilySchoolPro copy.app/FamilySchoolPro copy

错误原因：

框架（UIKit.framework）在低版本的设备上找不到，加载不上

解决方法：

h1, h2, h3, h4, h5, h6, p, blockquote { margin: 0; padding: 0 }
body { font-family: "Helvetica Neue", Helvetica, "Hiragino Sans GB", Arial, sans-serif; font-size: 13px; line-height: 18px; color: rgba(115, 115, 115, 1); background-color: rgba(255, 255, 255, 1); margin: 10px 13px }
table { margin: 10px 0 15px; border-collapse: collapse }
td, th { border: 1px solid rgba(221, 221, 221, 1); padding: 3px 10px }
th { padding: 5px 10px }
a { color: rgba(0, 105, 214, 1) }
a:hover { color: rgba(0, 80, 163, 1); text-decoration: none }
a img { border: none }
p { margin-bottom: 9px }
h1, h2, h3, h4, h5, h6 { color: rgba(64, 64, 64, 1); line-height: 36px }
h1 { margin-bottom: 18px; font-size: 30px }
h2 { font-size: 24px }
h3 { font-size: 18px }
h4 { font-size: 16px }
h5 { font-size: 14px }
h6 { font-size: 13px }
hr { margin: 0 0 19px; border-top: 0; border-right: 0; border-bottom: 1px solid rgba(204, 204, 204, 1); border-left: 0 }
blockquote { padding: 13px 13px 21px 15px; margin-bottom: 18px; font-family: georgia, serif; font-style: italic }
blockquote:before { content: "“"; font-size: 40px; margin-left: -10px; font-family: georgia, serif; color: rgba(238, 238, 238, 1) }
blockquote p { font-size: 14px; font-weight: 300; line-height: 18px; margin-bottom: 0; font-style: italic }
code, pre { font-family: Monaco, Andale Mono, Courier New, monospace }
code { background-color: rgba(254, 233, 204, 1); color: rgba(0, 0, 0, 0.75); padding: 1px 3px; font-size: 12px; -webkit-border-radius: 3px; -moz-border-radius: 3px; border-radius: 3px }
pre { display: block; padding: 14px; margin: 0 0 18px; line-height: 16px; font-size: 11px; border: 1px solid rgba(217, 217, 217, 1); white-space: pre-wrap; word-wrap: break-word }
pre code { background-color: rgba(255, 255, 255, 1); color: rgba(115, 115, 115, 1); font-size: 11px; padding: 0 }
sup { font-size: 0.83em; vertical-align: super; line-height: 0 }
* { -webkit-print-color-adjust: exact }
@media screen and (min-width: 914px) { body { width: 854px; margin: 10px auto } }
@media print { body, code, pre code, h1, h2, h3, h4, h5, h6 { color: rgba(0, 0, 0, 1) } table, pre { page-break-inside: avoid } }