when I use shader grass is disappearing

When i'm use nightvision grass dissapear. What's wrong?

 Shader "UnityFree/NightVision" {
     Properties {
         _MainTex ("Base (RGB)", 2D) = "white" {}
         _Brightness ("Brightness Amount", Float ) = 5
         _Bright ("Bright Areas", Float ) = 2.0
         
     }
     SubShader {
         Pass {
            
             CGPROGRAM
             #pragma vertex vert_img
             #pragma fragment frag
             #pragma fragmentoption ARB_precision_hint_fastest
             #include "UnityCG.cginc"
  
             uniform sampler2D _MainTex;
             uniform float _Brightness;
             uniform float _Bright;
             
             //uniform float _SaturationAmount;
             //uniform float _BrightnessAmount;
             //uniform float _ContrastAmount;
                    
             half4 frag (v2f_img i) : COLOR {
                 half4 tex = tex2D( _MainTex, i.uv );
                
                 // greyscale the pixel color values
                 //half4 grey = dot( half3( 0.222, 0.707, 0.071 ), tex );
                 half4 grey = dot( half3( 0.080, 0.040, 0.059 ), tex );
                
                 if(_Bright == 1.0 ) {
                     // white out bright spots
                     if( grey.x > 0.4 )
                         grey *= _Brightness;
                 }
                
                 // boost the green channel     
                 grey.y *= 1.8;
                 //grey.z *= 1.5; 
                 //grey.x *= 1.5;
                
                
                 return grey;        
             }
         ENDCG
         }
     }
 }

PS: How to create vision-range?(darkness is directly proportional to the distance of the player)